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/models.py

@@ -0,0 +1,47 @@
+import uuid
+
+from django.conf import settings
+from django.db import models
+
+
+class ChatConversation(models.Model):
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="chat_conversations",
+    )
+    title = models.CharField(max_length=255, blank=True, default="")
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-updated_at"]
+
+    def __str__(self):
+        return f"{self.title or 'Untitled'} ({self.user.username})"
+
+
+class ChatMessage(models.Model):
+    ROLE_CHOICES = [
+        ("user", "User"),
+        ("assistant", "Assistant"),
+        ("system", "System"),
+        ("tool", "Tool"),
+    ]
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    conversation = models.ForeignKey(
+        ChatConversation,
+        on_delete=models.CASCADE,
+        related_name="messages",
+    )
+    role = models.CharField(max_length=20, choices=ROLE_CHOICES)
+    content = models.TextField(blank=True, default="")
+    tool_calls = models.JSONField(null=True, blank=True)
+    tool_call_id = models.CharField(max_length=255, blank=True, null=True)
+    name = models.CharField(max_length=255, blank=True, null=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        ordering = ["created_at"]