Auto-stamp or migrate on startup

On boot, run_migrations() checks the Alembic revision:
- no revision + tables exist  → stamp head (db.sql was run manually)
- no revision + no tables     → upgrade head (fresh install)
- revision present            → upgrade head (no-op if already current)

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

app/main.py

@@ -3,7 +3,7 @@ from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 
-from app.db import check_db
+from app.db import check_db, run_migrations
 from app.routers import filaments, pricing, print_jobs, printers, todos
 
 logger = logging.getLogger("pops")
@@ -14,6 +14,7 @@ async def lifespan(app: FastAPI):
     ok, msg = check_db()
     if ok:
         logger.info("Database connection OK")
+        run_migrations()
     else:
         logger.warning("Database unreachable: %s", msg)
     yield