feat(chat): add LLM-powered travel agent with multi-provider support

Implement a full chat-based travel agent using LiteLLM for multi-provider
LLM support (OpenAI, Anthropic, Gemini, Ollama, Groq, Mistral, etc.).

Backend:
- New 'chat' Django app with ChatConversation and ChatMessage models
- Streaming SSE endpoint via StreamingHttpResponse
- 5 agent tools: search_places, list_trips, get_trip_details,
  add_to_itinerary, get_weather
- LiteLLM client wrapper with per-user API key retrieval
- System prompt with user preference context injection

Frontend:
- New /chat route with full-page chat UI (DaisyUI + Tailwind)
- Collapsible conversation sidebar with CRUD
- SSE streaming response display with tool call visualization
- Provider selector dropdown
- SSE proxy fix to stream text/event-stream without buffering
- Navbar link and i18n keys

backend/server/chat/admin.py

@@ -0,0 +1,17 @@
+from django.contrib import admin
+
+from .models import ChatConversation, ChatMessage
+
+
+@admin.register(ChatConversation)
+class ChatConversationAdmin(admin.ModelAdmin):
+    list_display = ("id", "user", "title", "updated_at", "created_at")
+    search_fields = ("title", "user__username")
+    list_filter = ("created_at", "updated_at")
+
+
+@admin.register(ChatMessage)
+class ChatMessageAdmin(admin.ModelAdmin):
+    list_display = ("id", "conversation", "role", "name", "created_at")
+    search_fields = ("conversation__id", "content", "name")
+    list_filter = ("role", "created_at")