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>

backend/app/schemas/ai_session.py

@@ -11,7 +11,12 @@ from pydantic import BaseModel, Field
 # ── Intake ──
 
 class AISessionCreateRequest(BaseModel):
-    """Start a new FlowPilot session."""
+    """Start a new FlowPilot or chat session."""
+    session_type: str = Field(
+        "guided",
+        pattern="^(guided|chat)$",
+        description="Session type: guided (FlowPilot) or chat (assistant)",
+    )
     intake_type: str = Field(
         "free_text",
         pattern="^(free_text|psa_ticket|screenshot|log_paste|combined)$",
@@ -192,6 +197,8 @@ class LinkTicketRequest(BaseModel):
 class AISessionSummary(BaseModel):
     """Compact session for list views."""
     id: UUID
+    session_type: str = "guided"
+    title: str | None = None
     status: str
     intake_type: str
     problem_summary: str | None = None
@@ -208,7 +215,7 @@ class AISessionSummary(BaseModel):
 
 
 class AISessionDetail(AISessionSummary):
-    """Full session detail with steps."""
+    """Full session detail with steps (guided) or messages (chat)."""
     intake_content: dict[str, Any]
     matched_flow_id: UUID | None = None
     match_score: float | None = None
@@ -220,10 +227,32 @@ class AISessionDetail(AISessionSummary):
     psa_connection_id: UUID | None = None
     ticket_data: dict[str, Any] | None = None
     steps: list[AISessionStepResponse] = []
+    conversation_messages: list[dict[str, Any]] = []  # Chat sessions store messages here
 
     model_config = {"from_attributes": True}
 
 
+# ── Chat session ──
+
+class ChatSessionCreateResponse(BaseModel):
+    """Response after creating a chat session on ai_sessions."""
+    session_id: UUID
+    session_type: str = "chat"
+    title: str
+    status: str = "active"
+
+
+class ChatMessageRequest(BaseModel):
+    """Send a message in a chat session."""
+    message: str = Field(..., min_length=1, max_length=8000)
+
+
+class ChatMessageResponse(BaseModel):
+    """AI response to a chat message."""
+    content: str
+    suggested_flows: list[dict[str, Any]] = []
+
+
 class AISessionSearchResult(BaseModel):
     """Lightweight session result for Command Palette / autocomplete."""
     id: UUID