v4: ingest signed relay notifications

[Phloraxx]

Scope

Seventh stacked PayGate v4 implementation slice. Depends on PR #48.

Implements

• reuses the proven v3 P-256 ECDSA device-signature model on direct SQLite
• device ID is SHA-256 of SubjectPublicKeyInfo DER
• request signature covers METHOD + path + timestamp + SHA256(raw body) with a 5-minute tolerance
• signature is explicitly bound to POST /api/v4/relay/events
• minimal v4 relay event JSON: schema version, source event ID, package, notification post time, title/text/big-text, optional untrusted amount hint
• v4.0 package allowlist is Paytm Business + Google Messages only; GPay remains rejected
• validates/bounds body and notification text before persistence
• only authenticated, valid allowlisted traffic refreshes device last_seen_at
• stores one SQLite relay_events row and normalizes provider wording server-side
• Paytm/Kotak normalized observations flow directly into the fail-closed matcher from PR v4: match payment observations safely #48
• .00 / unrelated notifications are stored as ignored Activity, never matched
• pre-enrollment notifications are ignored
• invalid/future notification post time falls back to server_received_at confidence rather than being trusted
• half-processed received relay rows can be safely resumed on retry
• completed duplicate events are idempotent and cannot emit duplicate payment transitions/webhooks
• no PocketBase, libgm, UTR/RRN, or provider-specific Android matching logic

Tests

End-to-end tests cover signed Paytm→paid, duplicate event idempotency, invalid signature, stale timestamp, tampered body, unsupported GPay, .00 ignore, pre-enrollment ignore, retrying a previously stored received event, and signed Kotak-through-Google-Messages→paid. Full internal/v4/... unit tests, race tests, vet and diff checks pass locally.

Safety

Still no production HTTP route registration and no Android rollout. This is an isolated v4 service package.