feat: tags personnalisés par utilisateur (user_id)

DB:
- tags.user_id REFERENCES users(id)
- UNIQUE(name, user_id) au lieu de UNIQUE(name)
- Migration v2.6: alter + recreate table

API (tous les endpoints tags):
- POST/PUT/DELETE /api/settings/tags: filtré par user_id
- GET /api/settings/tags/all: retourne uniquement les tags du user
- GET/POST /api/local-workspace/*/tags: scope user

→ Les tags de l'utilisateur 1 ne sont pas visibles par l'utilisateur 2

app/db.py

@@ -306,9 +306,11 @@ def init_db():
             -- v2.2.0: Tags system
             CREATE TABLE IF NOT EXISTS tags (
                 id INTEGER PRIMARY KEY AUTOINCREMENT,
-                name TEXT NOT NULL UNIQUE,
+                name TEXT NOT NULL,
                 color TEXT DEFAULT '#787774',
-                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+                user_id INTEGER NOT NULL DEFAULT 0 REFERENCES users(id),
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+                UNIQUE(name, user_id)
             );
 
             CREATE TABLE IF NOT EXISTS page_tags (
@@ -379,6 +381,19 @@ def init_db():
             )""")
         except sqlite3.OperationalError:
             pass
+        # v2.6: Tags per-user
+        try:
+            conn.execute("ALTER TABLE tags ADD COLUMN user_id INTEGER NOT NULL DEFAULT 0 REFERENCES users(id)")
+        except sqlite3.OperationalError:
+            pass
+        try:
+            # Recreate UNIQUE constraint for per-user tags
+            conn.execute("CREATE TABLE IF NOT EXISTS tags_new (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT NOT NULL, color TEXT DEFAULT '#787774', user_id INTEGER NOT NULL DEFAULT 0 REFERENCES users(id), created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP, UNIQUE(name, user_id))")
+            conn.execute("INSERT OR IGNORE INTO tags_new (id, name, color, user_id, created_at) SELECT id, name, color, COALESCE(user_id,0), created_at FROM tags")
+            conn.execute("DROP TABLE tags")
+            conn.execute("ALTER TABLE tags_new RENAME TO tags")
+        except sqlite3.OperationalError:
+            pass
         conn.commit()