[push][P3][operator] Firebase project + google-services.json + FCM secrets

[cestercian]

Part of the FCM push epic #24. Operator dashboard work — the only part that can't be done from a build environment. Unblocks P1 live delivery + P2b build.

Steps (~15 min)

1. Firebase console → Add project (or reuse an existing one) → add an Android app, package in.artistant.app (plus the flavor-suffixed ids if dev/staging are split: .dev, .staging).
2. Download google-services.json → app/src/<flavor>/ per flavor (or app/ for a shared one). It is gitignored — distribute via a CI secret / out-of-band. → hands off to P2b.
3. Project settings → Service accounts → generate a new private key (JSON) = the FCM v1 service account.
4. Set Supabase Edge Function secrets on both projects (dev + prod):
   • FCM_SERVICE_ACCOUNT = the service-account JSON.
   • FCM_PROJECT_ID = the Firebase project id.
5. Confirm app_settings.edge_function_base_url is set (the push enable-switch; dev first) so the DB triggers actually POST to send-push.

Acceptance

• google-services.json available to the build / CI for each flavor.
• Both secrets present on dev + prod (verify with a P1 test send returning non-503).

Blocks

• P1 live delivery, P2b build, P4 verification.