feat: postgres database integration

[Vuks69]

Summary

• Postgres database layer with connection pooling, env-var-based config, migrations, and CRUD operations for guilds, users, moderation logs, and caching
• In-memory cache
• Migration runner with atomic per-file transactions and embedded SQL migration files
• Background DB connection — gateway starts first, DB connects with retry in background (bot stays online if PG is down)
• Unit tests for cache (set/get/expiry/concurrency), command dispatch (exact match/bot filter/prefix), and env helper functions
• Docker compose with Postgres 16 service and healthcheck

Files changed

New

• internal/db/ — connection, migrations, cache, guild, user, moderation, types
• internal/db/migrations/001_init_schema.sql — initial schema (guilds, users, moderation_logs)
• internal/db/cache_test.go, internal/core/start_test.go, internal/db/connection_test.go

Modified

• internal/core/start.go — background DB init, context-aware shutdown, recover on goroutines
• internal/community/commands.go — adapted to new structure
• docker-compose.yml — added postgres service
• .env.example — DB configuration vars
• go.mod / go.sum — added lib/pq dependency