Telegram: Add PostgreSQL store and HA deployment mode

[ptone]

Summary

The Telegram integration (extras/scion-telegram/) currently only supports SQLite for state storage and runs exclusively as a go-plugin subprocess of the hub. Add PostgreSQL store support and a standalone service mode to enable HA deployment.

Current State

• ✅ Fully functional as go-plugin subprocess (managed by hub)
• ✅ Both long-polling and webhook inbound modes implemented
• ✅ SQLite store for group links, user mappings, conversation contexts, notification preferences
• ❌ No PostgreSQL store
• ❌ No standalone server mode (only go-plugin subprocess)

Work Required

• PostgreSQL store implementation (mirror the SQLite schema to Postgres, similar to Discord's store_postgres.go)
• Standalone gRPC server entry point in main.go (allow running as separate service, not just go-plugin subprocess)
• Self-managed plugin configuration support (hub connects to external Telegram service)
• State migration tooling (SQLite → Postgres for existing deployments upgrading to HA)
• Container deployment support (Dockerfile, Cloud Run / k8s config)
• Consider advisory lock or equivalent for webhook mode coordination (only one instance should hold the webhook registration)

Context

• Telegram supports both long-polling (outbound, works behind NAT) and webhook (requires public URL) modes
• Long-polling mode: only one instance can poll at a time (409 conflicts) — needs coordination similar to Discord's advisory lock
• Webhook mode: Telegram sends to a single URL, so a load balancer or single-instance constraint is needed
• Discord's store_postgres.go can serve as a reference implementation
• Part of broader chat integration admin UI initiative