refactor: split main.py into modules (config, database, models, utils, auth, runtime, maintenance)

main.py was ~3000 lines with models, routes, Docker ops, maintenance all mixed.
Split into 7 focused modules:
- config.py: env vars and constants
- database.py: SQLAlchemy engine, SessionLocal, Base, get_db
- models.py: ORM models and enums
- utils.py: logging, formatting, icon handling, misc helpers
- auth.py: password hashing, cookies, CSRF, user dependency
- runtime.py: all Docker operations, pool management, session lifecycle
- maintenance.py: cleanup loop, schema bootstrap, startup logic
- main.py: FastAPI app, middleware, all route handlers only

app/database.py

@@ -0,0 +1,20 @@
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
+
+from config import DATABASE_URL
+
+
+engine = create_engine(DATABASE_URL, pool_pre_ping=True)
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_db():
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()