feat(firebase): initial stable release v0.9.0

Firebase App component with launcher lifecycle and health check integration.

What's included:
- Config with ProjectID (FIREBASE_PROJECT_ID env var); credentials via ADC
- Provider interface exposing native *firebase.App directly
- Component interface: launcher.Component + health.Checkable + Provider
- New(logger, cfg) constructor for lifecycle registration via lc.Append
- Health check via GetUser("health-probe-non-existent") + auth.IsUserNotFound at LevelCritical
- No-op OnStop (Firebase Admin SDK has no Close method)

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

firebase.go

@@ -0,0 +1,91 @@
+package firebase
+
+import (
+	"context"
+	"fmt"
+
+	fb "firebase.google.com/go/v4"
+	"firebase.google.com/go/v4/auth"
+
+	"code.nochebuena.dev/go/health"
+	"code.nochebuena.dev/go/launcher"
+	"code.nochebuena.dev/go/logz"
+)
+
+// Provider is the minimal interface for consumers that only need the firebase App.
+type Provider interface {
+	App() *fb.App
+}
+
+// Component adds lifecycle management and health check to Provider.
+type Component interface {
+	launcher.Component
+	health.Checkable
+	Provider
+}
+
+// Config holds Firebase configuration.
+type Config struct {
+	// ProjectID is the Google Cloud Project ID.
+	ProjectID string `env:"FIREBASE_PROJECT_ID,required"`
+}
+
+type firebaseComponent struct {
+	cfg    Config
+	logger logz.Logger
+	app    *fb.App
+}
+
+// New returns a firebase Component. Call lc.Append(fb) to manage its lifecycle.
+func New(logger logz.Logger, cfg Config) Component {
+	return &firebaseComponent{cfg: cfg, logger: logger}
+}
+
+func (f *firebaseComponent) OnInit() error {
+	if f.cfg.ProjectID == "" {
+		return fmt.Errorf("firebase: ProjectID is required")
+	}
+	f.logger.Info("firebase: initializing app", "project_id", f.cfg.ProjectID)
+	app, err := fb.NewApp(context.Background(), &fb.Config{
+		ProjectID: f.cfg.ProjectID,
+	})
+	if err != nil {
+		f.logger.Error("firebase: failed to create app", err)
+		return fmt.Errorf("firebase: create app: %w", err)
+	}
+	f.app = app
+	return nil
+}
+
+func (f *firebaseComponent) OnStart() error {
+	f.logger.Info("firebase: app ready")
+	return nil
+}
+
+func (f *firebaseComponent) OnStop() error {
+	f.logger.Info("firebase: shutting down")
+	return nil
+}
+
+func (f *firebaseComponent) App() *fb.App { return f.app }
+
+func (f *firebaseComponent) HealthCheck(ctx context.Context) error {
+	if f.app == nil {
+		return fmt.Errorf("firebase: app not initialized")
+	}
+	authClient, err := f.app.Auth(ctx)
+	if err != nil {
+		return fmt.Errorf("firebase: get auth client: %w", err)
+	}
+	_, err = authClient.GetUser(ctx, "health-probe-non-existent")
+	if err != nil {
+		if auth.IsUserNotFound(err) {
+			return nil // expected: probe of non-existent UID succeeded
+		}
+		return err
+	}
+	return nil
+}
+
+func (f *firebaseComponent) Name() string          { return "firebase" }
+func (f *firebaseComponent) Priority() health.Level { return health.LevelCritical }