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

docker-compose.yml

@@ -32,6 +32,10 @@ services:
       DEBUG: "true"
       # Point to host Ollama (Mac default).  Override in .env if different.
       OLLAMA_URL: "${OLLAMA_URL:-http://host.docker.internal:11434}"
+      # Grok (xAI) — opt-in premium cloud backend
+      GROK_ENABLED: "${GROK_ENABLED:-false}"
+      XAI_API_KEY: "${XAI_API_KEY:-}"
+      GROK_DEFAULT_MODEL: "${GROK_DEFAULT_MODEL:-grok-3-fast}"
     extra_hosts:
       - "host.docker.internal:host-gateway"  # Linux compatibility
     networks: