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.

internal/database/database_test.go

@@ -10,7 +10,11 @@ import (
 	"git.unkin.net/unkin/encapi/pkg/models"
 )
 
-var testDB *DB
+// testDB is the shared throwaway database; testDSN reaches the same server.
+var (
+	testDB  *DB
+	testDSN string
+)
 
 func TestMain(m *testing.M) {
 	ctx := context.Background()
@@ -19,12 +23,13 @@ func TestMain(m *testing.M) {
 		// Docker unavailable: run so tests self-skip via requireDB.
 		os.Exit(m.Run())
 	}
-	db, err := New(dsn)
+	db, err := New(ctx, dsn, nil)
 	if err != nil {
 		terminate()
 		panic(err)
 	}
 	testDB = db
+	testDSN = dsn
 
 	code := m.Run()
 	db.Close()