feat(server): multi-DB backends with durable reactions/read receipts/channel state

[Cod-e-Codes]

Pull Request

Description

This PR delivers multi-database support and durable server-side state for reactions, read receipts, and channel membership (last channel), plus diagnostics, tests, and documentation updates.

Motivation

• Enable marchat to run against SQLite, PostgreSQL, or MySQL with one runtime configuration path (MARCHAT_DB_PATH).
• Keep SQL call sites largely backend-agnostic by rebinding placeholders where needed.
• Persist state that was previously transient (reactions, read receipts, channel choice) so reconnects and restarts preserve user experience.
• Give operators a clear server health picture across backends via -doctor / -doctor-json (dialect, DSN shape, connectivity).
• Document configuration and troubleshooting for the new backends.

What changed

Database runtime

• Added DB driver/dialect detection and initialization for:
  • SQLite file paths (default behavior)
  • PostgreSQL DSNs (postgres:// / postgresql://)
  • MySQL DSNs (mysql: / mysql://)
• Added per-handle dialect tracking and rebinding helpers:
  • setDBDialect, getDBDialect
  • rebindQuery, dbExec, dbQuery, dbQueryRow
• Added dialect-aware SQL helpers for upsert/insert-ignore patterns used by durable state tables.

Durable message state

• Added durable message-state helpers:
  • Persist/load reactions
  • Persist/load read receipts
  • Persist/load user channel (last channel)
• Wired client read loop paths to persist:
  • reactions
  • read receipts
  • channel changes (join/leave/fallback to general)

Diagnostics (internal/doctor)

• Server -doctor / -doctor-json now aligns with multi-DB configuration:
  • Reports detected DB dialect and driver (db_dialect check).
  • Validates connection string format (Postgres via pgconn.ParseConfig, MySQL via mysql.ParseDSN, SQLite non-empty path) (db_connection_string check).
  • Pings the configured database using the same driver/DSN split as the server (db_ping check; replaces the previous SQLite-only sqlite_ping check ID).
• SQLite-only filesystem checks (parent directory exists/writable) run only when the dialect is SQLite.
• Blank imports for MySQL and pgx stdlib drivers so sql.Open works for doctor pings.

Tests

• server/db_dialect_test.go — DSN detection and Postgres placeholder rebinding.
• server/message_state_test.go — reaction target ID and removal path (IsRemoval: true); empty-table / first-boot behavior for reactions, read receipts, and user channel loaders.
• internal/doctor/db_checks_test.go — detectDBTarget and validateConnectionString coverage.

Documentation

• README.md — multi-backend setup, DSN examples, troubleshooting; Diagnostics (-doctor) section updated for server dialect validation and DB ping.
• ARCHITECTURE.md — Diagnostics (internal/doctor) section updated to describe dialect, connection-string validation, and DB/TLS ping checks.
• ROADMAP.md — completion tracking updates (as applicable on this branch).
• TESTING.md — backend coverage and testing notes (as applicable on this branch).

Notes for reviewers / operators

• JSON consumers: If anything parses -doctor-json and keyed off sqlite_ping, update to db_ping (and optionally read db_dialect / db_connection_string).
• CI: Default go test ./... still uses SQLite (in-memory/temp); Postgres/MySQL remain manual or separate CI jobs unless you add them.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Breaking change (protocol/binary): None intended for the WebSocket protocol. Doctor JSON: check ID sqlite_ping was renamed to db_ping and new checks were added; treat as a minor compatibility note for automation only.

Checklist

• My code builds and passes all tests/CI
• I have commented my code, particularly in hard-to-understand areas
• I have updated documentation as needed
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have assigned reviewers