chore(main): remove SQLite ALTER TABLE migration shim

PostgreSQL is the actual DB; create_all handles schema creation.
The SQLite workaround added in the previous commit is not needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/main.py

@@ -21,13 +21,6 @@ async def lifespan(app: FastAPI):
     # Startup - ensure tables exist
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
-        # SQLite migration: add accepted_at column if missing
-        try:
-            await conn.exec_driver_sql(
-                "ALTER TABLE sec_filings ADD COLUMN accepted_at TIMESTAMP"
-            )
-        except Exception:
-            pass  # Column already exists
     # Start overlay batch scheduler (graceful no-op if apscheduler not installed)
     try:
         from app.services.overlay.scheduler import start_scheduler