fastapi-sqlalchemy-pg-catalog/app: clearer DATABASE_URL error + async-safe create_all

Two nits from the Copilot review:

* DATABASE_URL: `os.environ["DATABASE_URL"]` raised a bare KeyError
  at import time, which surfaces as a noisy traceback in container
  logs with no hint about what's missing. Switched to os.getenv with
  an explicit RuntimeError that names the env var and gives a sample
  URL format.

* lifespan: `Base.metadata.create_all(engine)` is synchronous
  psycopg2 I/O running inside an async lifespan, blocking uvicorn's
  event loop until pg_class probe + any CREATE TABLE round-trips
  complete. Switched to `await asyncio.to_thread(...)` so the loop
  stays responsive. For this minimal repro the difference is small,
  but the pattern is the right FastAPI shape for any startup that
  touches a sync DB driver.

Signed-off-by: Akash Kumar <meakash7902@gmail.com>

Author: AkashKumar7902

fastapi-sqlalchemy-pg-catalog/app/main.py

@@ -16,6 +16,7 @@
 (simple-query path, ``dispatchBySQLHash``).
 """
 
+import asyncio
 import logging
 import os
 import sys
@@ -32,7 +33,12 @@
 )
 log = logging.getLogger("repro")
 
-DATABASE_URL = os.environ["DATABASE_URL"]
+DATABASE_URL = os.getenv("DATABASE_URL")
+if not DATABASE_URL:
+    raise RuntimeError(
+        "DATABASE_URL is required (e.g. postgresql+psycopg2://user:pass@host:5432/db). "
+        "Set it in docker-compose env or in the host shell before launching uvicorn."
+    )
 # SQL echo is INTENTIONALLY on by default — this is a sample for
 # demonstrating the dispatcher's simple-Query catalog path, and seeing
 # the actual SQLAlchemy queries (pg_catalog.version, pg_class probe,
@@ -59,7 +65,13 @@ class Project(Base):
 @asynccontextmanager
 async def lifespan(_: FastAPI):
     log.info("startup: running Base.metadata.create_all (pg_class probe expected)")
-    Base.metadata.create_all(engine)
+    # create_all does synchronous psycopg2 I/O. Offload to a thread so
+    # uvicorn's event loop stays responsive (otherwise any other
+    # async work scheduled on startup would block until the pg_class
+    # probe + any CREATE TABLE round-trips complete). For this minimal
+    # repro the difference is small, but the pattern is the right
+    # FastAPI shape for any startup that touches a sync DB driver.
+    await asyncio.to_thread(Base.metadata.create_all, engine)
     log.info("startup: create_all complete")
     yield