Add LLM chat integration with streaming responses via Ollama

Phase 4: Full chat system with SSE streaming, note-aware context, and
conversation persistence.

Backend:
- Migration 0005: conversations + messages tables with FKs and indexes
- Conversation/Message SQLAlchemy models with relationships
- LLM service: ensure_model (auto-pull on startup), stream_chat (NDJSON),
  generate_completion, fetch_url_content (HTML stripping), build_context
  (keyword extraction, related note search, URL content injection)
- Chat service: conversation CRUD, save_response_as_note,
  summarize_conversation_as_note
- Chat routes blueprint: 9 endpoints including SSE streaming for messages,
  save/summarize as note, Ollama model listing
- Auto-pull llama3.1 model on app startup (non-blocking)

Frontend:
- apiStreamPost: SSE client using fetch + ReadableStream
- Chat Pinia store with streaming state management
- ChatView: dedicated /chat page with conversation sidebar + message thread
- ChatPanel: slide-out panel with contextNoteId from current route
- ChatMessage: markdown-rendered message bubble with "Save as Note" action
- Updated AppHeader with Chat nav link + panel toggle button
- Updated App.vue to mount ChatPanel with route-derived context
- Added /chat and /chat/:id routes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/types/chat.ts

@@ -0,0 +1,26 @@
+export interface Message {
+  id: number;
+  conversation_id: number;
+  role: "system" | "user" | "assistant";
+  content: string;
+  context_note_id: number | null;
+  created_at: string;
+}
+
+export interface Conversation {
+  id: number;
+  title: string;
+  model: string;
+  message_count: number;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface ConversationDetail extends Conversation {
+  messages: Message[];
+}
+
+export interface OllamaModel {
+  name: string;
+  size: number;
+}