feat: add Grok (xAI) as opt-in premium backend with monetization

- Add GrokBackend class in src/timmy/backends.py with full sync/async
  support, health checks, usage stats, and cost estimation in sats
- Add consult_grok tool to Timmy's toolkit for proactive Grok queries
- Extend cascade router with Grok provider type for failover chain
- Add Grok Mode toggle card to Mission Control dashboard (HTMX live)
- Add "Ask Grok" button on chat input for direct Grok queries
- Add /grok/* routes: status, toggle, chat, stats endpoints
- Integrate Lightning invoice generation for Grok usage monetization
- Add GROK_ENABLED, XAI_API_KEY, GROK_DEFAULT_MODEL, GROK_MAX_SATS_PER_QUERY,
  GROK_FREE config settings via pydantic-settings
- Update .env.example and docker-compose.yml with Grok env vars
- Add 21 tests covering backend, tools, and route endpoints (all green)

Local-first ethos preserved: Grok is premium augmentation only,
disabled by default, and Lightning-payable when enabled.

https://claude.ai/code/session_01FygwN8wS8J6WGZ8FPb7XGV

src/config.py

@@ -24,13 +24,22 @@ class Settings(BaseSettings):
     # "airllm"  — always use AirLLM (requires pip install ".[bigbrain]")
     # "auto"    — use AirLLM on Apple Silicon if airllm is installed,
     #             fall back to Ollama otherwise
-    timmy_model_backend: Literal["ollama", "airllm", "auto"] = "ollama"
+    timmy_model_backend: Literal["ollama", "airllm", "grok", "auto"] = "ollama"
 
     # AirLLM model size when backend is airllm or auto.
     # Larger = smarter, but needs more RAM / disk.
     # 8b  ~16 GB  |  70b  ~140 GB  |  405b  ~810 GB
     airllm_model_size: Literal["8b", "70b", "405b"] = "70b"
 
+    # ── Grok (xAI) — opt-in premium cloud backend ────────────────────────
+    # Grok is a premium augmentation layer — local-first ethos preserved.
+    # Only used when explicitly enabled and query complexity warrants it.
+    grok_enabled: bool = False
+    xai_api_key: str = ""
+    grok_default_model: str = "grok-3-fast"
+    grok_max_sats_per_query: int = 200
+    grok_free: bool = False  # Skip Lightning invoice when user has own API key
+
     # ── Spark Intelligence ────────────────────────────────────────────────
     # Enable/disable the Spark cognitive layer.
     # When enabled, Spark captures swarm events, runs EIDOS predictions,