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

.env.example

@@ -30,6 +30,15 @@
 # 8b  ~16 GB RAM  |  70b  ~140 GB RAM  |  405b  ~810 GB RAM
 # AIRLLM_MODEL_SIZE=70b
 
+# ── Grok (xAI) — premium cloud augmentation ──────────────────────────────────
+# Enable Grok as an opt-in premium backend for frontier reasoning.
+# Local-first ethos is preserved — Grok only activates when explicitly enabled.
+# GROK_ENABLED=false
+# XAI_API_KEY=xai-...
+# GROK_DEFAULT_MODEL=grok-3-fast
+# GROK_MAX_SATS_PER_QUERY=200
+# GROK_FREE=false
+
 # ── L402 Lightning secrets ───────────────────────────────────────────────────
 # HMAC secret for invoice verification.  MUST be changed in production.
 # Generate with: python3 -c "import secrets; print(secrets.token_hex(32))"