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

internal/db/queries/source.sql

@@ -0,0 +1,24 @@
+-- name: UpsertSource :one
+INSERT INTO sources (id, name, lang, is_nsfw)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (id) DO UPDATE
+    SET name    = EXCLUDED.name,
+        lang    = EXCLUDED.lang,
+        is_nsfw = EXCLUDED.is_nsfw
+RETURNING *;
+
+-- name: ListSources :many
+SELECT * FROM sources ORDER BY id;
+
+-- name: GetSourceByID :one
+SELECT * FROM sources WHERE id = $1;
+
+-- name: GetSourceMeta :one
+SELECT value FROM source_meta
+WHERE source_id = $1 AND key = $2;
+
+-- name: SetSourceMeta :exec
+INSERT INTO source_meta (source_id, key, value)
+VALUES ($1, $2, $3)
+ON CONFLICT (source_id, key) DO UPDATE
+    SET value = EXCLUDED.value;