Replace SELECT 1 warmup with full database file read to warm OS page cache

Reading the entire DB file into OS page cache eliminates the 8-14s
cold-start penalty on the first ORM query (users/templates/api_keys).
Previous SELECT 1 only warmed the connection pool, not the file cache.

gns3server/db/tasks.py

@@ -99,14 +99,17 @@ async def connect_to_db(app: FastAPI) -> None:
             return row[0] if row else "unknown"
         wal_mode = await _verify_conn.run_sync(_check_wal)
         log.info(f"SQLite journal mode: {wal_mode} {'✅' if wal_mode and wal_mode.upper() == 'WAL' else '❌ will cause database contention'}")
-    # Warm up the connection pool and ORM cache so the first API request
-    # doesn't pay the cold-start penalty (observed: 6-8s first DB query)
-    async with engine.connect() as _warmup:
-        def _warmup_query(conn):
-            from sqlalchemy import text
-            conn.execute(text("SELECT 1"))
-        await _warmup.run_sync(_warmup_query)
-        log.info("Database connection pool warmed up")
+    # Warm up the OS page cache by reading the entire database file.
+    # The first API query suffers 8-14s cold-start penalty when the file
+    # isn't in cache (SQLite + OS page cache combined).
+    try:
+        db_file_size = os.path.getsize(db_path)
+        with open(db_path, "rb") as f:
+            while f.read(1024 * 1024):  # 1MB chunks
+                pass
+        log.info(f"Database warmed up ({db_file_size / 1024:.0f} KB)")
+    except OSError as e:
+        log.warning(f"Could not warm up database file: {e}")
     alembic_cfg = config.Config()
     alembic_cfg.set_main_option("script_location", "gns3server:db_migrations")
     #alembic_cfg.set_main_option('sqlalchemy.url', db_url)