Detect course roots per branch and add a category layer above them

The scanner no longer assumes courses sit at a fixed depth. It walks
each branch and marks a folder is_course when it holds media directly
or its children read as chapters (01_, 1. , Module 2, Week 3);
everything above that is a category, and every folder still gets a
course_id pointing at its nearest course for cheap rollups. An empty
.course/.category marker file overrides the heuristic per folder.

Also:
- Roll up progress for the whole library in two grouped queries
  instead of one prefix-matched query per course/folder card.
- Store folders/videos/quizzes paths as utf8mb4_bin so folders that
  differ only in case (CCNP Notes vs CCNP NOtes) aren't merged by a
  case-insensitive UNIQUE key.
- migrateLibrarySchema() brings an older install's tables up to date
  at scan time, so there's no separate migration step.
- Library UI gains a categories view alongside shelves/catalog; the
  continue-watching strip's play button now seeks to the saved
  position on open.

Claude-Session: https://claude.ai/code/session_01FfMPJK7TQV18JhFv6yTKev

README.md

@@ -14,8 +14,8 @@ views switch without page reloads and browser back/forward still work:
 | Route | Screen |
 | --- | --- |
 | `#/` | home — resume card, stats, in progress, last scan |
-| `#/library`, `#/library/<shelf>` | every course, as shelves or a filtered catalog |
-| `#/folder/N` | a course or section: sections + content |
+| `#/library`, `#/library/<shelf>` | categories, shelves or a flat catalog of every course |
+| `#/folder/N` | a category (its courses), or a course/section (its contents) |
 | `#/watch/N` | player, playlist, section quizzes |
 | `#/quiz/N` | take and grade a quiz |
 | `#/search/Q` | courses, videos and quizzes matching `Q` |
@@ -25,11 +25,13 @@ Only `stream.php` (the mp4 bytes) and `captions.php` (subtitles as WebVTT) are
 plain HTTP resources.
 
 - Recursively scans a videos directory for `.mp4` files and mirrors the folder
-  hierarchy (courses → sections → videos, any nesting depth).
+  hierarchy (categories → courses → sections → videos, any nesting depth), and
+  works out **where a course starts** rather than assuming a depth.
 - Per-user **resume**: log in from any device and continue where you left off.
 - **Progress tracking at every level** — each folder shows completed/total and a
   percentage bar computed over its whole subtree.
-- "Continue watching" strip, auto-play next video, playback speed, keyboard
+- "Continue watching" strip — its play button opens the player *and* starts the
+  video at the saved position — auto-play next video, playback speed, keyboard
   shortcuts (Plyr), completed checkmarks, natural sorting (`Video2` before `Video10`).
 - **Subtitles**: a video with a sibling `.srt` file (`Lesson.mp4` +
   `Lesson.srt`) gets a CC button in the player — the file is converted to
@@ -38,28 +40,94 @@ plain HTTP resources.
 - **Reset progress** for one video, one quiz, a whole course/section, or the
   entire account.
 - Course and section listings switch between **grid and list** layout, and the
-  library between **shelves and catalog**; both choices are remembered.
+  library between **categories, shelves and catalog**; both choices are remembered.
+- **Categories** are the folders that sit above the courses — the library opens
+  on them, and each one totals the courses beneath it.
 - **Shelves** group courses by how far you are: in progress, finished, not
-  started. The app has no course categories, so progress is what sorts them.
+  started. The sidebar links straight to each one.
 - Seekable streaming via HTTP Range requests; files are addressed by DB id only
   (no path traversal).
 
 ## Layout on disk
 
+Put whatever shape your material already has under `VIDEOS_ROOT`. There is no
+convention to follow and nothing to rename: the scanner works out which folder
+is a course, per branch, and everything else follows from that.
+
 ```
 videos/
-├── Course 1/
-│   ├── Intro/
-│   │   ├── Video1.mp4
-│   │   └── Video2.mp4
-│   └── Module 2/
-│       └── Advanced/
-│           └── lesson.mp4
-└── Course 2/
-    └── ...
+├── Cloud/                                  category
+│   └── AWS Networking/                     course  (children are numbered)
+│       ├── 01 - Welcome/
+│       │   └── Intro.mp4
+│       └── 02 - Creating a VPC/
+│           └── Subnets.mp4
+├── Firewalls/                              category
+│   └── Palo-Alto/                          category
+│       └── Palo_Alto_Firewall_Essentials/  course
+│           └── 01_Welcome/…
+└── System Admin/                           category
+    └── Red Hat … EX457/                    course  (videos sit right in it)
+        ├── 01 Introduction.mp4
+        └── 02 Inventories.mp4
 ```
 
-Top-level folders become courses on the home page.
+### Where a course starts
+
+Course roots do not sit at a fixed depth — one library can hold
+`Category/<course>`, `Category/Vendor/<course>` and
+`Category/Vendor/Exam/<course>` at once — so the scanner decides per branch,
+walking down from the top:
+
+| The folder… | is a |
+| --- | --- |
+| has no videos or quizzes anywhere under it | neither — it is skipped |
+| holds videos or quizzes directly | **course** |
+| has children whose names read as chapters (`01_`, `1. `, `Module 2`, `Week 3`) | **course** |
+| anything else | **category** — look one level deeper |
+
+Everything under a course root is one of its sections. The result is stored as
+`folders.is_course` / `folders.course_id` at scan time, so the screens are a
+lookup rather than a guess, and re-running the scan re-derives it.
+
+### Overriding it
+
+The rule reads chapter numbering, so it gets a course wrong when the sections
+carry plain names (`Course/Introduction`, `Course/Advanced` looks like two
+courses). Drop an **empty marker file** in the folder and the next scan obeys it
+instead of guessing:
+
+| File | Effect |
+| --- | --- |
+| `.course` | this folder is a course; everything under it is a section |
+| `.category` | the courses are further down — keep looking one level deeper |
+
+```bash
+touch "videos/Cat/Unnumbered Course/.course"     # one course, not two
+touch "videos/Cat/Exam Bundle/.category"         # each child is its own course
+```
+
+A marker settles only the folder it sits on. Once a course root is found,
+everything below it is a section, so to push a course root *deeper*, mark the
+folder above it `.category`. A `.category` on a folder that holds videos itself
+is ignored — those videos have to belong to something.
+
+Two consequences worth knowing:
+
+- **Progress means something.** Shelves, the home page's "in progress" strip and
+  every percentage are per course. Counting a whole category as one course
+  instead buries twenty watched videos in a denominator of three thousand, and
+  everything reads 0 % forever.
+- **A folder of PDFs is not a course.** Anything with no `.mp4` and no quiz file
+  anywhere beneath it never appears in the library at all.
+
+### Folder names are case-sensitive
+
+`CCNP Notes` and `CCNP NOtes` are two directories on a Linux filesystem and the
+app treats them as two, because `folders.path`, `videos.path` and `quizzes.path`
+are stored `utf8mb4_bin`. Under the default case-insensitive collation the
+`UNIQUE` key on `path` merged them, and the second tree's videos were never
+indexed. The search box still matches without regard to case.
 
 ## Setup
 
@@ -106,7 +174,9 @@ Top-level folders become courses on the home page.
 
    Rescan whenever you add/remove files. Re-scans are idempotent: existing
    videos keep their IDs (and everyone's progress); files that disappeared are
-   hidden, not deleted, so progress survives if they come back.
+   hidden, not deleted, so progress survives if they come back. Each scan also
+   re-derives which folders are courses, and brings an older install's tables up
+   to date first, so there is no separate migration step.
 
 5. Open the site, create an account, and watch.
 
@@ -235,6 +305,10 @@ next scan creates it.
   Completion is sticky — rewatching doesn't un-complete it.
 - Folder progress is computed over the folder's entire subtree using a path
   prefix match, with partial credit for videos in progress.
+- Every course is rolled up in **two queries for the whole library**, grouped by
+  `folders.course_id`, and a folder's children in two more — not two per card.
+  A library of a few hundred courses would otherwise spend a page load on
+  several hundred prefix-matched queries.
 - Video duration is read at scan time by parsing the mp4's `mvhd` header in
   pure PHP, so listings show durations before anyone plays the video. If a file
   can't be parsed, the browser reports its duration on first play instead.