feat: Add OAuth token and database metrics (Phases 3-4)

Complete Prometheus instrumentation for OAuth token operations
and additional database operations to populate empty dashboard panels.

OAuth Token Metrics (Phase 4):
- unified_verifier.py:
  * Token validation cache hits/misses
  * JWT verification success/failure/error
  * Introspection validation results
  * Audience validation failures
- context_helper.py:
  * Token exchange cache hits/misses
  * RFC 8693 exchange success/error

Database Metrics (Phase 3 completion):
- storage.py:
  * get_refresh_token() with timing
  * delete_refresh_token() with timing
  * All operations record duration and success/error status

These metrics populate the following dashboard panels:
- Token Validations (by method and result)
- Token Cache Hit Rate
- Token Exchange Operations
- Database Operations (refresh token CRUD)
- Database Operation Duration

Part of PR #295 - Complete metrics instrumentation

nextcloud_mcp_server/auth/storage.py

@@ -432,40 +432,45 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        async with aiosqlite.connect(self.db_path) as db:
-            async with db.execute(
-                """
-                SELECT encrypted_token, expires_at, flow_type, token_audience,
-                       provisioned_at, provisioning_client_id, scopes
-                FROM refresh_tokens WHERE user_id = ?
-                """,
-                (user_id,),
-            ) as cursor:
-                row = await cursor.fetchone()
-
-        if not row:
-            logger.debug(f"No refresh token found for user {user_id}")
-            return None
-
-        (
-            encrypted_token,
-            expires_at,
-            flow_type,
-            token_audience,
-            provisioned_at,
-            provisioning_client_id,
-            scopes_json,
-        ) = row
-
-        # Check expiration
-        if expires_at is not None and expires_at < time.time():
-            logger.warning(
-                f"Refresh token for user {user_id} has expired (expired at {expires_at})"
-            )
-            await self.delete_refresh_token(user_id)
-            return None
-
+        start_time = time.time()
         try:
+            async with aiosqlite.connect(self.db_path) as db:
+                async with db.execute(
+                    """
+                    SELECT encrypted_token, expires_at, flow_type, token_audience,
+                           provisioned_at, provisioning_client_id, scopes
+                    FROM refresh_tokens WHERE user_id = ?
+                    """,
+                    (user_id,),
+                ) as cursor:
+                    row = await cursor.fetchone()
+
+            if not row:
+                logger.debug(f"No refresh token found for user {user_id}")
+                duration = time.time() - start_time
+                record_db_operation("sqlite", "select", duration, "success")
+                return None
+
+            (
+                encrypted_token,
+                expires_at,
+                flow_type,
+                token_audience,
+                provisioned_at,
+                provisioning_client_id,
+                scopes_json,
+            ) = row
+
+            # Check expiration
+            if expires_at is not None and expires_at < time.time():
+                logger.warning(
+                    f"Refresh token for user {user_id} has expired (expired at {expires_at})"
+                )
+                await self.delete_refresh_token(user_id)
+                duration = time.time() - start_time
+                record_db_operation("sqlite", "select", duration, "success")
+                return None
+
             decrypted_token = self.cipher.decrypt(encrypted_token).decode()
             scopes = json.loads(scopes_json) if scopes_json else None
 
@@ -473,6 +478,9 @@ class RefreshTokenStorage:
                 f"Retrieved refresh token for user {user_id} (flow_type: {flow_type})"
             )
 
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "select", duration, "success")
+
             return {
                 "refresh_token": decrypted_token,
                 "expires_at": expires_at,
@@ -484,6 +492,8 @@ class RefreshTokenStorage:
                 "scopes": scopes,
             }
         except Exception as e:
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "select", duration, "error")
             logger.error(f"Failed to decrypt refresh token for user {user_id}: {e}")
             return None
 
@@ -578,25 +588,34 @@ class RefreshTokenStorage:
         if not self._initialized:
             await self.initialize()
 
-        async with aiosqlite.connect(self.db_path) as db:
-            cursor = await db.execute(
-                "DELETE FROM refresh_tokens WHERE user_id = ?",
-                (user_id,),
-            )
-            await db.commit()
-            deleted = cursor.rowcount > 0
+        start_time = time.time()
+        try:
+            async with aiosqlite.connect(self.db_path) as db:
+                cursor = await db.execute(
+                    "DELETE FROM refresh_tokens WHERE user_id = ?",
+                    (user_id,),
+                )
+                await db.commit()
+                deleted = cursor.rowcount > 0
 
-        if deleted:
-            logger.info(f"Deleted refresh token for user {user_id}")
-            await self._audit_log(
-                event="delete_refresh_token",
-                user_id=user_id,
-                auth_method="offline_access",
-            )
-        else:
-            logger.debug(f"No refresh token to delete for user {user_id}")
+            duration = time.time() - start_time
+            record_db_operation("sqlite", "delete", duration, "success")
 
-        return deleted
+            if deleted:
+                logger.info(f"Deleted refresh token for user {user_id}")
+                await self._audit_log(
+                    event="delete_refresh_token",
+                    user_id=user_id,
+                    auth_method="offline_access",
+                )
+            else:
+                logger.debug(f"No refresh token 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_user_ids(self) -> list[str]:
         """