Add a QR code endpoint for PLM addresses

GET /qr/{address} renders the address as a PNG QR code (qrcode[pil]),
gated by a bech32-shaped regex since it's otherwise unauthenticated —
the address itself isn't sensitive, but this keeps it from being used
as an arbitrary text-to-QR service.

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

app/main.py

@@ -15,6 +15,7 @@ import app.rounds.confirmation  # noqa: F401  (registers the "payout" confirmati
 import app.withdrawals.confirmation  # noqa: F401  (registers the "withdrawal" confirmation handler)
 from app.api.routes.admin import router as admin_router
 from app.api.routes.bets import router as bets_router
+from app.api.routes.qr import router as qr_router
 from app.api.routes.users import router as users_router
 from app.api.routes.withdrawals import router as withdrawals_router
 from app.auth.routes import router as auth_router
@@ -65,6 +66,7 @@ app.include_router(users_router)
 app.include_router(bets_router)
 app.include_router(withdrawals_router)
 app.include_router(admin_router)
+app.include_router(qr_router)
 
 
 @app.exception_handler(Exception)