Harden auth, add subtitles, vendor all assets locally

- Session cookies are HttpOnly + SameSite=Lax (Secure on HTTPS), with
  strict session-ID mode; the CSRF token from GET api/auth.php must now
  accompany every state-changing request (X-CSRF-Token header, or a csrf
  body field for the sendBeacon progress save), rotated at login.
- Failed sign-ins are throttled per username and IP via login_attempts
  (created lazily, so existing installs need no migration): 5 failures a
  username / 20 an IP inside 15 minutes answer 429 with Retry-After.
- A video's sibling .srt now appears as a CC button in the player;
  captions.php converts it to WebVTT (comma timestamps, legacy encodings,
  and the already-VTT files with out-of-spec 6-digit fractions).
- Plyr and Inter ship in assets/vendor/ instead of coming from CDNs, and
  anime.js is gone along with all scripted animations and the motion
  setting — the app now runs with no internet access at all.
- .htaccess denies direct access to videos/, includes/, config.php and
  schema.sql, so video URLs cannot bypass login on Apache.

Claude-Session: https://claude.ai/code/session_01SePf61iazhCLJBefdRLBDm

captions.php

@@ -0,0 +1,29 @@
+<?php
+// Serves a video's subtitles as WebVTT, converted on the fly from the .srt
+// file sitting next to the .mp4. Like stream.php, the file is addressed by
+// database ID only — the client never supplies a filesystem path.
+require_once __DIR__ . '/includes/auth.php';
+require_once __DIR__ . '/includes/functions.php';
+
+if (currentUserId() === null) {
+    http_response_code(403);
+    exit('Forbidden');
+}
+
+$video = getVideo((int)($_GET['id'] ?? 0));
+$file  = $video ? srtFilePath($video) : null;
+if ($file === null) {
+    http_response_code(404);
+    exit('No subtitles for this video');
+}
+
+$srt = file_get_contents($file);
+if ($srt === false) {
+    http_response_code(404);
+    exit('No subtitles for this video');
+}
+
+session_write_close();
+header('Content-Type: text/vtt; charset=utf-8');
+header('Cache-Control: private, max-age=3600');
+echo srtToVtt($srt);