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/alembic/env.py

@@ -20,6 +20,8 @@ from app.models.ai_suggestion import AISuggestion  # noqa: F401
 from app.models.kb_import import KBImport, KBImportNode  # noqa: F401
 from app.models.script_template import ScriptCategory, ScriptTemplate, ScriptGeneration  # noqa: F401
 from app.models.psa_connection import PsaConnection  # noqa: F401
+from app.models.ai_session import AISession  # noqa: F401
+from app.models.ai_session_step import AISessionStep  # noqa: F401
 from app.models.psa_post_log import PsaPostLog  # noqa: F401
 from app.models.psa_member_mapping import PsaMemberMapping  # noqa: F401
 from app.core.config import settings