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/components/AppHeader.vue

@@ -2,6 +2,10 @@
 import { useTheme } from "@/composables/useTheme";
 
 const { theme, toggleTheme } = useTheme();
+
+const emit = defineEmits<{
+  toggleChatPanel: [];
+}>();
 </script>
 
 <template>
@@ -11,6 +15,10 @@ const { theme, toggleTheme } = useTheme();
       <div class="nav-links">
         <router-link to="/notes" class="nav-link">Notes</router-link>
         <router-link to="/tasks" class="nav-link">Tasks</router-link>
+        <router-link to="/chat" class="nav-link">Chat</router-link>
+        <button class="btn-chat-panel" @click="emit('toggleChatPanel')" title="Open chat panel">
+          &#x1F4AC;
+        </button>
         <button class="theme-toggle" @click="toggleTheme" :title="`Switch to ${theme === 'dark' ? 'light' : 'dark'} mode`">
           {{ theme === "dark" ? "\u2600" : "\u263E" }}
         </button>
@@ -51,6 +59,19 @@ const { theme, toggleTheme } = useTheme();
 .nav-link:hover {
   color: var(--color-primary);
 }
+.btn-chat-panel {
+  background: none;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  padding: 0.25rem 0.5rem;
+  cursor: pointer;
+  font-size: 1rem;
+  color: var(--color-text);
+  line-height: 1;
+}
+.btn-chat-panel:hover {
+  background: var(--color-bg-card);
+}
 .theme-toggle {
   background: none;
   border: 1px solid var(--color-border);