feat: add collections management and study coach features

- Implemented collection repository with CRUD operations for user collections.
- Created routes for managing collections, including listing, creating, updating, and deleting collections.
- Added sharing functionality for exams, allowing users to share and clone exams.
- Developed a study coach service that analyzes exam results and provides study tips based on weak areas.
- Introduced frontend components for managing collections, sharing exams, and displaying study coach insights.
- Enhanced user experience with loading states and error handling in shared components.

backend/src/db/migrations.ts

@@ -53,6 +53,19 @@ export function runMigrations(db: Database.Database): void {
     CREATE INDEX IF NOT EXISTS idx_exam_results_user ON exam_results(user_id);
   `);
 
+  // ── Add share_token and is_public columns if missing ──
+  const examCols = db.prepare(`PRAGMA table_info(saved_exams)`).all() as { name: string }[];
+  if (!examCols.some((c) => c.name === 'share_token')) {
+    db.exec(`ALTER TABLE saved_exams ADD COLUMN share_token TEXT`);
+  }
+  if (!examCols.some((c) => c.name === 'is_public')) {
+    db.exec(`ALTER TABLE saved_exams ADD COLUMN is_public INTEGER DEFAULT 0`);
+  }
+
+  db.exec(`
+    CREATE UNIQUE INDEX IF NOT EXISTS idx_saved_exams_share ON saved_exams(share_token);
+  `);
+
   // ── Add role column if missing (existing databases) ──
   const cols = db.prepare(`PRAGMA table_info(users)`).all() as {
     name: string;
@@ -61,15 +74,53 @@ export function runMigrations(db: Database.Database): void {
     db.exec(`ALTER TABLE users ADD COLUMN role TEXT NOT NULL DEFAULT 'user'`);
   }
 
-  // ── Add storage_quota_bytes if missing ──
-  if (!cols.some((c) => c.name === "storage_quota_bytes")) {
-    db.exec(`ALTER TABLE users ADD COLUMN storage_quota_bytes INTEGER NOT NULL DEFAULT 1073741824`);
-  }
+    // ── Add storage_quota_bytes if missing ──
+    if (!cols.some((c) => c.name === "storage_quota_bytes")) {
+      db.exec(`ALTER TABLE users ADD COLUMN storage_quota_bytes INTEGER NOT NULL DEFAULT 1073741824`);
+    }
 
-  // ── Add plan if missing ──
-  if (!cols.some((c) => c.name === "plan")) {
-    db.exec(`ALTER TABLE users ADD COLUMN plan TEXT NOT NULL DEFAULT 'free'`);
-  }
+    // ── Add plan if missing ──
+    if (!cols.some((c) => c.name === "plan")) {
+      db.exec(`ALTER TABLE users ADD COLUMN plan TEXT NOT NULL DEFAULT 'free'`);
+    }
+
+    // ── Add is_favorite to saved_exams if missing ──
+    const examSavedCols = db.prepare(`PRAGMA table_info(saved_exams)`).all() as { name: string }[];
+    if (!examSavedCols.some((c) => c.name === 'is_favorite')) {
+      db.exec(`ALTER TABLE saved_exams ADD COLUMN is_favorite INTEGER DEFAULT 0`);
+    }
+
+    // ── Add tags to saved_exams if missing ──
+    if (!examSavedCols.some((c) => c.name === 'tags')) {
+      db.exec(`ALTER TABLE saved_exams ADD COLUMN tags TEXT DEFAULT '[]'`);
+    }
+
+    // ── Add collection_id to saved_exams if missing ──
+    if (!examSavedCols.some((c) => c.name === 'collection_id')) {
+      db.exec(`ALTER TABLE saved_exams ADD COLUMN collection_id INTEGER REFERENCES collections(id) ON DELETE SET NULL`);
+    }
+
+    // ── Add share_mode to saved_exams if missing ──
+    if (!examSavedCols.some((c) => c.name === 'share_mode')) {
+      db.exec(`ALTER TABLE saved_exams ADD COLUMN share_mode TEXT DEFAULT 'clone'`);
+    }
+
+    // ── Add cloned_from_share_token to saved_exams if missing ──
+    if (!examSavedCols.some((c) => c.name === 'cloned_from_share_token')) {
+      db.exec(`ALTER TABLE saved_exams ADD COLUMN cloned_from_share_token TEXT`);
+    }
+
+    // ── Collections table ──
+    db.exec(`
+      CREATE TABLE IF NOT EXISTS collections (
+        id          INTEGER PRIMARY KEY AUTOINCREMENT,
+        user_id     INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        name        TEXT NOT NULL,
+        created_at  INTEGER DEFAULT (unixepoch()),
+        updated_at  INTEGER DEFAULT (unixepoch())
+      );
+      CREATE INDEX IF NOT EXISTS idx_collections_user ON collections(user_id);
+    `);
 
   // ── Promote first user to admin if no admins exist ──
   const adminCount = db