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

go.mod

@@ -4,10 +4,18 @@ go 1.26.2
 
 require (
 	github.com/PuerkitoBio/goquery v1.12.0
+	github.com/golang-migrate/migrate/v4 v4.19.1
+	github.com/jackc/pgx/v5 v5.9.2
 	golang.org/x/time v0.15.0
 )
 
 require (
 	github.com/andybalholm/cascadia v1.3.3 // indirect
+	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa // indirect
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
 	golang.org/x/net v0.52.0 // indirect
+	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/text v0.35.0 // indirect
 )