Scoped DB role + own theme colour (port log E17)

- Split database.py into a runtime engine (scoped `kds` DB role, used for
  all request handling) and a migration engine (privileged `kitchen` role,
  used only at startup to create KDS's own tables and ALTER kitchen_settings)
  — KDS previously shared kitchen's full-access DB credential wholesale
- Dispose both engines on shutdown (main.py)
- Fix theme colour clash: KDS was accidentally seeded with kitchen's teal
  (#0d9488) instead of its own colour — now #ea580c (orange), regenerated
  PWA icons to match

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/main.py

@@ -5,7 +5,7 @@ from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
 
-from database import engine, Base
+from database import engine, runtime_engine, Base
 
 logging.basicConfig(
     level=logging.INFO,
@@ -34,7 +34,9 @@ async def _run(name, coro):
 
 @asynccontextmanager
 async def lifespan(app: FastAPI):
-    # Create any SQLAlchemy-mapped KDS tables (idempotent)
+    # Create any SQLAlchemy-mapped KDS tables (idempotent). Uses the
+    # privileged migration engine (`kitchen` role) — the scoped `kds` role
+    # used for request handling can't CREATE TABLE.
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
 
@@ -51,6 +53,7 @@ async def lifespan(app: FastAPI):
 
     await stop_signalr_listener()
     await engine.dispose()
+    await runtime_engine.dispose()
 
 
 app = FastAPI(