feat: Phase 2 — database layer

- PostgreSQL schema: sources, manga, chapters, pages, source_meta with indexes
- golang-migrate runner with embedded SQL via go:embed (pgx5:// scheme)
- sqlc-generated type-safe queries for all tables (pgx/v5 native)
- Config package with all env vars including TTL durations
- Wire DB init and config into cmd/server/main.go

TODO.md

@@ -6,7 +6,7 @@ Detailed checklists are in each phase doc under `docs/`.
 ---
 
 - [x] **Phase 1 — Core Framework** → `docs/phase1-core-framework.md`
-- [ ] **Phase 2 — Database Layer** → `docs/phase2-database.md`
+- [x] **Phase 2 — Database Layer** → `docs/phase2-database.md`
 - [ ] **Phase 3 — Base Source Implementations (68 bases)** → `docs/phase3-bases.md`
 - [ ] **Phase 4 — Standalone Sources (555 sources)** → `docs/phase4-standalone.md`
 - [ ] **Phase 5 — HTTP API** → `docs/phase5-api.md`