Add quiz support: parse *_quiz_solved.txt files, take and grade quizzes

Scanner indexes quiz files found next to videos ([&] questions, "- "
options, "-- Correct" markers; _notsolved twins ignored). Quizzes appear
in the folder content list interleaved with videos by filename order.
New api/quiz.php serves questions without answers and grades attempts
server-side; per-user best/last score and attempts stored in
quiz_progress. Folder/course progress now aggregates videos + quizzes
with partial credit for partial scores.

schema.sql

@@ -41,6 +41,41 @@ CREATE TABLE IF NOT EXISTS videos (
     INDEX idx_videos_folder (folder_id)
 ) ENGINE=InnoDB;
 
+-- One row per "*_quiz_solved.txt" file found next to the videos.
+-- Questions/options are parsed from the file on demand; only the count is
+-- cached here for progress math.
+CREATE TABLE IF NOT EXISTS quizzes (
+    id            INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    folder_id     INT UNSIGNED NOT NULL,
+    name          VARCHAR(255) NOT NULL,   -- filename without the _quiz_solved.txt suffix
+    filename      VARCHAR(255) NOT NULL,
+    path          VARCHAR(700) NOT NULL UNIQUE,
+    num_questions INT UNSIGNED NOT NULL DEFAULT 0,
+    sort_order    INT UNSIGNED NOT NULL DEFAULT 0,
+    is_missing    TINYINT(1) NOT NULL DEFAULT 0,
+    created_at    TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    CONSTRAINT fk_quizzes_folder FOREIGN KEY (folder_id)
+        REFERENCES folders(id) ON DELETE CASCADE,
+    INDEX idx_quizzes_folder (folder_id)
+) ENGINE=InnoDB;
+
+-- Per-user quiz results. completed = every question answered correctly
+-- in a single attempt (sticky).
+CREATE TABLE IF NOT EXISTS quiz_progress (
+    user_id         INT UNSIGNED NOT NULL,
+    quiz_id         INT UNSIGNED NOT NULL,
+    best_score      INT UNSIGNED NOT NULL DEFAULT 0,
+    last_score      INT UNSIGNED NOT NULL DEFAULT 0,
+    total_questions INT UNSIGNED NOT NULL DEFAULT 0,
+    attempts        INT UNSIGNED NOT NULL DEFAULT 0,
+    completed       TINYINT(1) NOT NULL DEFAULT 0,
+    updated_at      TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
+        ON UPDATE CURRENT_TIMESTAMP,
+    PRIMARY KEY (user_id, quiz_id),
+    CONSTRAINT fk_qp_user FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
+    CONSTRAINT fk_qp_quiz FOREIGN KEY (quiz_id) REFERENCES quizzes(id) ON DELETE CASCADE
+) ENGINE=InnoDB;
+
 -- Per-user playback state: resume position + completion flag.
 CREATE TABLE IF NOT EXISTS video_progress (
     user_id          INT UNSIGNED NOT NULL,