fix(auth): Store app passwords locally for multi-user BasicAuth background sync

Previously, the multi-user BasicAuth mode attempted to retrieve app passwords
via OAuth client_credentials grant, which Nextcloud OIDC doesn't support.

This fix implements local storage for app passwords:
- Add app_passwords table via Alembic migration (002)
- Add store/get/delete methods to RefreshTokenStorage
- Add management API endpoints for app password provisioning:
  - POST /api/v1/users/{user_id}/app-password
  - GET /api/v1/users/{user_id}/app-password
  - DELETE /api/v1/users/{user_id}/app-password
- Update oauth_sync.py to read from local storage
- Update Astrolabe to send app passwords to MCP server after validation
- Add app-hook to configure mcp_server_url in Nextcloud

The flow is now:
1. User creates app password in Nextcloud Security settings
2. User enters it in Astrolabe Personal Settings
3. Astrolabe validates against Nextcloud, then sends to MCP server
4. MCP server stores encrypted app password locally
5. Background sync uses locally stored password

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/auth/storage.py

@@ -1240,6 +1240,180 @@ class RefreshTokenStorage:
 
         return deleted
 
+    # ============================================================================
+    # App Password Storage (multi-user BasicAuth mode)
+    # ============================================================================
+
+    async def store_app_password(
+        self,
+        user_id: str,
+        app_password: str,
+    ) -> None:
+        """
+        Store encrypted app password for background sync (multi-user BasicAuth mode).
+
+        Args:
+            user_id: Nextcloud user ID
+            app_password: Nextcloud app password to store
+        """
+        if not self._initialized:
+            await self.initialize()
+
+        if not self.cipher:
+            raise RuntimeError(
+                "Encryption key not configured. "
+                "Set TOKEN_ENCRYPTION_KEY for app password storage."
+            )
+
+        encrypted_password = self.cipher.encrypt(app_password.encode())
+        now = int(time.time())
+
+        start_time = time.time()
+        try:
+            async with aiosqlite.connect(self.db_path) as db:
+                await db.execute(
+                    """
+                    INSERT OR REPLACE INTO app_passwords
+                    (user_id, encrypted_password, created_at, updated_at)
+                    VALUES (
+                        ?,
+                        ?,
+                        COALESCE((SELECT created_at FROM app_passwords WHERE user_id = ?), ?),
+                        ?
+                    )
+                    """,
+                    (user_id, encrypted_password, user_id, now, now),
+                )
+                await db.commit()
+
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "insert", duration, "success")
+            logger.info(f"Stored app password for user {user_id}")
+
+        except Exception:
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "insert", duration, "error")
+            raise
+
+        # Audit log
+        await self._audit_log(
+            event="store_app_password",
+            user_id=user_id,
+            auth_method="app_password",
+        )
+
+    async def get_app_password(self, user_id: str) -> Optional[str]:
+        """
+        Retrieve and decrypt app password for a user.
+
+        Args:
+            user_id: Nextcloud user ID
+
+        Returns:
+            Decrypted app password, or None if not found
+        """
+        if not self._initialized:
+            await self.initialize()
+
+        if not self.cipher:
+            raise RuntimeError(
+                "Encryption key not configured. "
+                "Set TOKEN_ENCRYPTION_KEY for app password retrieval."
+            )
+
+        start_time = time.time()
+        try:
+            async with aiosqlite.connect(self.db_path) as db:
+                async with db.execute(
+                    "SELECT encrypted_password FROM app_passwords WHERE user_id = ?",
+                    (user_id,),
+                ) as cursor:
+                    row = await cursor.fetchone()
+
+            if not row:
+                logger.debug(f"No app password found for user {user_id}")
+                duration = time.time() - start_time
+                record_db_operation("sqlite", "select", duration, "success")
+                return None
+
+            encrypted_password = row[0]
+            decrypted_password = self.cipher.decrypt(encrypted_password).decode()
+
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "select", duration, "success")
+            logger.debug(f"Retrieved app password for user {user_id}")
+
+            return decrypted_password
+
+        except Exception as e:
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "select", duration, "error")
+            logger.error(f"Failed to decrypt app password for user {user_id}: {e}")
+            return None
+
+    async def delete_app_password(self, user_id: str) -> bool:
+        """
+        Delete app password for a user.
+
+        Args:
+            user_id: Nextcloud user ID
+
+        Returns:
+            True if password was deleted, False if not found
+        """
+        if not self._initialized:
+            await self.initialize()
+
+        start_time = time.time()
+        try:
+            async with aiosqlite.connect(self.db_path) as db:
+                cursor = await db.execute(
+                    "DELETE FROM app_passwords WHERE user_id = ?",
+                    (user_id,),
+                )
+                await db.commit()
+                deleted = cursor.rowcount > 0
+
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "delete", duration, "success")
+
+            if deleted:
+                logger.info(f"Deleted app password for user {user_id}")
+                await self._audit_log(
+                    event="delete_app_password",
+                    user_id=user_id,
+                    auth_method="app_password",
+                )
+            else:
+                logger.debug(f"No app password to delete for user {user_id}")
+
+            return deleted
+
+        except Exception:
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "delete", duration, "error")
+            raise
+
+    async def get_all_app_password_user_ids(self) -> list[str]:
+        """
+        Get list of all user IDs with stored app passwords.
+
+        Returns:
+            List of user IDs
+        """
+        if not self._initialized:
+            await self.initialize()
+
+        async with aiosqlite.connect(self.db_path) as db:
+            async with db.execute(
+                "SELECT user_id FROM app_passwords ORDER BY updated_at DESC"
+            ) as cursor:
+                rows = await cursor.fetchall()
+
+        user_ids = [row[0] for row in rows]
+        logger.debug(f"Found {len(user_ids)} users with app passwords")
+        return user_ids
+
 
 async def generate_encryption_key() -> str:
     """