feat: unified sessions — merge assistant chat into ai_sessions table

Add session_type ('guided'|'chat') and title columns to ai_sessions,
enabling both FlowPilot guided sessions and assistant chat sessions to
live in a single table. This is the foundation for a unified session
history and consistent UX across both interaction modes.

Backend:
- Migration 066: session_type + title columns
- unified_chat_service: chat sessions on ai_sessions with same AI/RAG
- POST /ai-sessions supports session_type='chat' creation
- POST /ai-sessions/{id}/chat for chat messages
- DELETE /ai-sessions/{id} for session deletion
- session_type filter on GET /ai-sessions

Frontend:
- AssistantChatPage rewired to aiSessionsApi (no more assistantChatApi)
- /assistant/:sessionId route for deep-linking
- Session history: type filter pills (All/Guided/Chat), type icons
- Dashboard: both types shown with correct routing and icons
- Fixed glass-border → border-default in dashboard components

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

frontend/src/hooks/useFlowPilotSession.ts

@@ -69,6 +69,8 @@ export function useFlowPilotSession(): UseFlowPilotSession {
 
       setSession({
         id: result.session_id,
+        session_type: 'guided',
+        title: null,
         status: result.status,
         intake_type: intake.intake_type,
         intake_content: intake.intake_content,
@@ -89,6 +91,7 @@ export function useFlowPilotSession(): UseFlowPilotSession {
         psa_connection_id: intake.psa_connection_id ?? null,
         ticket_data: null,
         steps: [firstStep],
+        conversation_messages: [],
       })
       setAllSteps([firstStep])
       setCurrentStep(firstStep)