feat(search): add PostgreSQL FTS on AI sessions with Command Palette integration

- Migration: add generated tsvector column + GIN index on ai_sessions (problem_summary, resolution_summary, escalation_reason, problem_domain)
- Backend: wire FTS into list_sessions q param; add GET /ai-sessions/search endpoint returning AISessionSearchResult (registered before /{session_id} to avoid UUID routing conflict)
- Frontend: add AISessionSearchResult type, aiSessionsApi.search() method, and Command Palette group "FlowPilot Sessions" using Zap icon navigating to /pilot/:id

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

backend/app/schemas/ai_session.py

@@ -190,3 +190,14 @@ class AISessionDetail(AISessionSummary):
     steps: list[AISessionStepResponse] = []
 
     model_config = {"from_attributes": True}
+
+
+class AISessionSearchResult(BaseModel):
+    """Lightweight session result for Command Palette / autocomplete."""
+    id: UUID
+    problem_summary: str | None = None
+    problem_domain: str | None = None
+    status: str
+    created_at: datetime
+
+    model_config = {"from_attributes": True}