Rebuild payment API with PocketBase and Go

[Phloraxx]

Summary

Replaces the Fastify/SQLite prototype with the reviewed Go + PocketBase implementation and embedded React operator UI.

Key changes:

• durable exact-amount DDM allocation and quarantine
• exact bank-SMS matching with RRN/source-event idempotency and stale-message protection
• optional read-only Google Messages/libgm connector
• durable outgoing webhook queue with HMAC signing/retries
• PocketBase auth/realtime/admin/backup foundation
• single non-root production container with persistent /app/pb_data
• legacy /api/webhook compatibility route gated behind explicit migration config

Validation

Local release gate passed:

• npm ci + npm audit --audit-level=high
• frontend TypeScript check + production build
• gofmt + git diff --check
• go test -count=1 ./...
• go test -race -count=1 ./internal/...
• go vet ./...
• staticcheck
• govulncheck: no reachable vulnerabilities
• fresh-volume production container smoke tests
• container-replacement persistence test
• independent review found no discrete correctness issue

GitHub Actions CI run #45 also passed every step, including the production Docker build, for head 9a065bf9eb0e6446640af2b3cb57902d2defbd10.

Production cutover blocker

Do not merge/deploy yet. Dokploy's persistent volume is already saved, but production still needs new strong PAYGATE_API_KEY and SMS_WEBHOOK_SECRET values before the new server can pass startup validation. The old one-character WEBHOOK_SECRET is retained only for the explicitly enabled temporary legacy relay route during migration.