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/ChatMessage.vue

@@ -0,0 +1,113 @@
+<script setup lang="ts">
+import { computed } from "vue";
+import { renderMarkdown } from "@/utils/markdown";
+import type { Message } from "@/types/chat";
+
+const props = defineProps<{
+  message: Message;
+  isStreaming?: boolean;
+}>();
+
+const emit = defineEmits<{
+  saveAsNote: [messageId: number];
+}>();
+
+const rendered = computed(() => renderMarkdown(props.message.content));
+
+const roleLabel = computed(() => {
+  switch (props.message.role) {
+    case "user":
+      return "You";
+    case "assistant":
+      return "Assistant";
+    default:
+      return props.message.role;
+  }
+});
+</script>
+
+<template>
+  <div class="chat-message" :class="`role-${message.role}`">
+    <div class="message-header">
+      <span class="role-label">{{ roleLabel }}</span>
+      <div class="message-actions" v-if="message.role === 'assistant' && !isStreaming">
+        <button class="btn-save" @click="emit('saveAsNote', message.id)" title="Save as note">
+          Save as Note
+        </button>
+      </div>
+    </div>
+    <div class="message-content prose" v-html="rendered"></div>
+    <div
+      v-if="message.context_note_id"
+      class="context-badge"
+    >
+      <router-link :to="`/notes/${message.context_note_id}`">
+        Note #{{ message.context_note_id }}
+      </router-link>
+    </div>
+  </div>
+</template>
+
+<style scoped>
+.chat-message {
+  padding: 0.75rem 1rem;
+  border-radius: 8px;
+  margin-bottom: 0.5rem;
+}
+.role-user {
+  background: var(--color-bg-secondary);
+}
+.role-assistant {
+  background: var(--color-bg-card);
+  border: 1px solid var(--color-border);
+}
+.message-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  margin-bottom: 0.25rem;
+}
+.role-label {
+  font-size: 0.8rem;
+  font-weight: 600;
+  color: var(--color-text-muted);
+  text-transform: uppercase;
+}
+.message-content {
+  font-size: 0.95rem;
+  line-height: 1.5;
+  word-break: break-word;
+}
+.message-content :deep(p:last-child) {
+  margin-bottom: 0;
+}
+.message-actions {
+  display: flex;
+  gap: 0.5rem;
+}
+.btn-save {
+  font-size: 0.75rem;
+  padding: 0.15rem 0.5rem;
+  background: transparent;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  color: var(--color-text-secondary);
+  cursor: pointer;
+}
+.btn-save:hover {
+  background: var(--color-primary);
+  color: #fff;
+  border-color: var(--color-primary);
+}
+.context-badge {
+  margin-top: 0.5rem;
+  font-size: 0.8rem;
+}
+.context-badge a {
+  color: var(--color-primary);
+  text-decoration: none;
+}
+.context-badge a:hover {
+  text-decoration: underline;
+}
+</style>