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/App.vue

@@ -1,13 +1,38 @@
 <script setup lang="ts">
+import { ref, computed } from "vue";
+import { useRoute } from "vue-router";
 import AppHeader from "@/components/AppHeader.vue";
+import ChatPanel from "@/components/ChatPanel.vue";
 import ToastNotification from "@/components/ToastNotification.vue";
 import { useTheme } from "@/composables/useTheme";
 
 useTheme();
+
+const route = useRoute();
+const chatPanelOpen = ref(false);
+
+const contextNoteId = computed(() => {
+  const id = route.params.id;
+  if (!id) return null;
+  const path = route.path;
+  if (path.startsWith("/notes/") || path.startsWith("/tasks/")) {
+    return Number(id);
+  }
+  return null;
+});
+
+function toggleChatPanel() {
+  chatPanelOpen.value = !chatPanelOpen.value;
+}
 </script>
 
 <template>
-  <AppHeader />
+  <AppHeader @toggle-chat-panel="toggleChatPanel" />
   <router-view />
+  <ChatPanel
+    v-if="chatPanelOpen"
+    :context-note-id="contextNoteId"
+    @close="chatPanelOpen = false"
+  />
   <ToastNotification />
 </template>