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/

docs/adr/ADR-001-sdk-health-check-pattern.md

@@ -0,0 +1,60 @@
+# ADR-001: SDK Health Check via Known-Nonexistent UID Probe
+
+**Status:** Accepted
+**Date:** 2026-03-18
+
+## Context
+
+The firebase-admin-go SDK wraps gRPC and HTTP internally. There is no explicit "ping" or
+"connection check" method on the `firebase.App` or `auth.Client` types. Health checking
+requires making a real API call that exercises the actual SDK communication path.
+
+Two naive approaches have drawbacks:
+
+1. **Check for a known real user**: Requires a stable test user to exist in production
+   Firebase. This is a maintenance burden and a security concern.
+2. **String-match on the error message**: Coupling to SDK error message text is fragile;
+   internal messages change across SDK versions without notice.
+
+## Decision
+
+The health check calls `authClient.GetUser(ctx, "health-probe-non-existent")` with a UID
+that is guaranteed not to exist. The expected outcome is a "user not found" error. The SDK
+provides a typed predicate for this:
+
+```go
+_, err = authClient.GetUser(ctx, "health-probe-non-existent")
+if err != nil {
+    if auth.IsUserNotFound(err) {
+        return nil // expected: the probe succeeded, service is reachable
+    }
+    return err // unexpected error: network failure, auth error, etc.
+}
+return nil
+```
+
+`auth.IsUserNotFound(err)` is an official SDK helper that inspects the error's underlying
+type, not its message string. It is stable across SDK versions.
+
+A "not found" response proves:
+- The Firebase project is reachable.
+- Authentication (ADC or service account key) is valid.
+- The Auth service responded correctly.
+
+Any other error (permission denied, network timeout, invalid project) is treated as a health
+failure and propagated to the health framework.
+
+## Consequences
+
+**Positive:**
+- The probe exercises the actual authentication and network path.
+- `auth.IsUserNotFound` is a stable, typed check that does not depend on error messages.
+- No real user is needed; the probe UID can never collide with a real account.
+- The component is marked `health.LevelCritical` — if the probe fails, the service is
+  considered unhealthy.
+
+**Negative:**
+- Every health check invocation makes a live API call to Firebase. Under high-frequency
+  health polling, this generates traffic. Health check intervals should be configured
+  conservatively (e.g. every 30 seconds, not every second).
+- The probe UID `"health-probe-non-existent"` is hardcoded. It is not configurable.