feat(skel): db package with embedded migrations stub

Establishes the embed.FS and migrations/ layout up front. Migrate()
returns ErrNotConfigured intentionally — the real goose runner lands in
M2 alongside Postgres work, avoiding a toolchain bump to Go 1.24/1.25
through goose's transitive modernc.org/libc dependency chain.

internal/db/db.go

@@ -0,0 +1,33 @@
+package db
+
+import (
+	"embed"
+	"errors"
+)
+
+//go:embed migrations/*.sql
+var migrationsFS embed.FS
+
+// MigrationsFS exposes the embedded migration files. The concrete migration
+// runner (goose) will be wired in M2 when Postgres work begins; this package
+// intentionally keeps the embed + a placeholder surface so the file layout
+// and `go:embed` directive are established from day one.
+func MigrationsFS() embed.FS {
+	return migrationsFS
+}
+
+// ErrNotConfigured is returned by Migrate until the real runner is wired.
+var ErrNotConfigured = errors.New("db: migrations not yet configured (wired in M2)")
+
+// Migrate is a placeholder. It validates that the embedded migrations load
+// and then returns ErrNotConfigured so callers can detect the stub.
+func Migrate() error {
+	entries, err := migrationsFS.ReadDir("migrations")
+	if err != nil {
+		return err
+	}
+	if len(entries) == 0 {
+		return errors.New("db: no migration files found")
+	}
+	return ErrNotConfigured
+}