Adopt golib/pg for migrations and pool construction

encapi's schema was a cumulative IF NOT EXISTS blob re-executed inline on every
start: it grows forever, records nothing, and cannot express a change that is
not a fresh CREATE. golib owns that mechanism now, so encapi keeps the SQL and
drops the runner.

- Move the DDL verbatim into migrations/0001_init.sql, embedded via
  migrations.FS. It stays IF NOT EXISTS-guarded, so the first start against the
  live database re-runs it as a no-op and only lands the schema_migrations row.
- Build the pool with pg.NewMigrated under the advisory lock named
  encapi-migrations, and delete the inline migrate(). database.New now takes a
  context and a logger; main.go hands it the signal context so a start blocked
  on the migration lock still dies on SIGTERM.
- Render the DSN with pg.DSN. The env var contract is untouched — the fields are
  still resolved by internal/config, because encapi defaults DBUSER and DBNAME
  to "encapi" where pg.DSNFromEnv treats both as required.
- Guard the set: embedded files must match migrations/, every CREATE must be
  idempotent, the derived lock key is pinned, and a container test proves the
  adoption path over a database that predates schema_migrations.
- Plumb GOPRIVATE=git.unkin.net for the first cross-repo Go dependency:
  exported by the Makefile, set in the Dockerfile and the woodpecker Go steps,
  documented in the README.

cmd/encapi/main.go

@@ -27,7 +27,10 @@ func main() {
 		os.Exit(1)
 	}
 
-	db, err := database.New(cfg.DatabaseDSN())
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	db, err := database.New(ctx, cfg.DatabaseDSN(), slog.Default())
 	if err != nil {
 		slog.Error("connect database", "err", err)
 		os.Exit(1)
@@ -40,9 +43,6 @@ func main() {
 
 	srv := server.New(db, distro.New(cfg.DistroAPIURL), cfg.WriteToken)
 
-	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
-	defer stop()
-
 	if err := srv.ListenAndServe(ctx, cfg.ListenAddr); err != nil {
 		slog.Error("server", "err", err)
 		os.Exit(1)