feat(google): support service-account auth for TTS + Imagen, fix false availability

Google's TTS and Imagen tools advertised service-account auth
(GOOGLE_APPLICATION_CREDENTIALS) but only ever authenticated with an API
key string, so users with a service-account JSON could not use either tool.
google_tts.get_status() also over-reported availability when the JSON was
set, then failed at execute() — a silent-availability bug.

Separately, both hand-rolled _load_dotenv parsers kept inline comments as
values, so after `cp .env.example .env` every keyed tool falsely reported
"available" with no real credentials.

Changes:
- Add tools/google_credentials.py: lazy google-auth Bearer-token helper.
- google_tts: authenticate via Cloud TTS Bearer token when only a service
  account is configured; make get_status() honest.
- google_imagen: route service-account auth to Vertex AI
  ({location}-aiplatform.googleapis.com) with project/location resolution,
  alongside the existing AI Studio API-key path.
- Fix both _load_dotenv parsers to strip inline comments (quote-aware).
- Add google-auth to requirements; document the new env vars in .env.example.
- .gitignore: never commit GCP service-account key files.

Verified locally with a real service account: TTS produced a valid MP3 and
Imagen produced a valid 1408x768 PNG via Vertex AI. Existing test suite
passes (2 unrelated pre-existing failures only).

Closes #131

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -8,6 +8,11 @@ FAL_KEY=                     # FLUX images, Google Veo video, Kling video, MiniM
 # --- Google (one key unlocks image gen + TTS) ---
 GOOGLE_API_KEY=              # Google Imagen images, Google Cloud TTS (700+ voices, 50+ languages)
                              # Get one at https://aistudio.google.com/apikey
+# Alternative to the API key: service-account JSON auth.
+# TTS uses Cloud Text-to-Speech; Imagen routes to Vertex AI.
+GOOGLE_APPLICATION_CREDENTIALS=   # path to a service-account JSON key file
+GOOGLE_CLOUD_PROJECT=             # GCP project id (required for Imagen via Vertex AI)
+GOOGLE_CLOUD_LOCATION=            # Vertex AI region, default us-central1
 
 # --- Voice ---
 ELEVENLABS_API_KEY=          # TTS narration, music generation, sound effects