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>

frontend/src/types/ai-session.ts

@@ -141,3 +141,11 @@ export interface AISessionDetail extends AISessionSummary {
   ticket_data: Record<string, unknown> | null
   steps: AISessionStepResponse[]
 }
+
+export interface AISessionSearchResult {
+  id: string
+  problem_summary: string | null
+  problem_domain: string | null
+  status: string
+  created_at: string
+}