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

sqlc.yaml

@@ -0,0 +1,13 @@
+version: "2"
+sql:
+  - engine: "postgresql"
+    schema: "internal/db/migrations/"
+    queries: "internal/db/queries/"
+    gen:
+      go:
+        package: "queries"
+        out: "internal/db/queries"
+        sql_package: "pgx/v5"
+        emit_json_tags: true
+        emit_db_tags: true
+        emit_pointers_for_null_types: true