feat(ai-session): add FlowPilot AI-powered troubleshooting sessions

Implements Phase 1 of the FlowPilot-First pivot — the core AI session
experience where engineers describe a problem and FlowPilot guides them
through structured diagnosis with selectable options, free-text escape
hatches, and auto-generated documentation on resolution.

Backend: AISession + AISessionStep models, FlowPilot Engine (LLM
orchestration with structured JSON output), Flow Matching Engine v1
(semantic + keyword + recency scoring), 8 API endpoints with auth,
rate limiting, and AI quota enforcement.

Frontend: Intake screen, conversational session view with sidebar,
step cards with options/actions/resolution suggestions, resolve/escalate
modals, documentation view with rating, session history integration,
and /pilot route with sidebar navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/router.py

@@ -21,6 +21,7 @@ from app.api.endpoints import integrations
 from app.api.endpoints import onboarding
 from app.api.endpoints import branding
 from app.api.endpoints import supporting_data
+from app.api.endpoints import ai_sessions
 
 api_router = APIRouter()
 
@@ -67,3 +68,4 @@ api_router.include_router(integrations.router)
 api_router.include_router(onboarding.router)
 api_router.include_router(branding.router)
 api_router.include_router(supporting_data.router)
+api_router.include_router(ai_sessions.router)