Add multi-user auth, background generation, and chat UX improvements

Phase 5: Multi-user authentication with session cookies, bcrypt passwords,
first-user-is-admin pattern, per-user data isolation, backup/restore,
Docker Swarm production stack with secrets and network isolation.

Phase 5.1: Chat UX improvements:
- Background generation architecture (GenerationBuffer + asyncio task)
  with SSE fan-out, reconnect support, and periodic DB flushes
- LLM-generated conversation titles (first exchange + every 10th message)
- Stop generation button with cancel_event and partial content preservation
- Relative timestamps in sidebar (5m ago, 3h ago, then dates)
- Empty chat auto-cleanup on navigation away
- Save-as-note uses LLM for title generation, tags notes with "chat"
- Summarize-as-note also tags with "chat"

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

src/fabledassistant/auth.py

@@ -0,0 +1,41 @@
+import functools
+
+from quart import g, jsonify, session
+
+from fabledassistant.services.auth import get_user_by_id
+
+
+def login_required(f):
+    @functools.wraps(f)
+    async def decorated(*args, **kwargs):
+        user_id = session.get("user_id")
+        if not user_id:
+            return jsonify({"error": "Authentication required"}), 401
+        user = await get_user_by_id(user_id)
+        if not user:
+            session.clear()
+            return jsonify({"error": "Authentication required"}), 401
+        g.user = user
+        return await f(*args, **kwargs)
+    return decorated
+
+
+def admin_required(f):
+    @functools.wraps(f)
+    async def decorated(*args, **kwargs):
+        user_id = session.get("user_id")
+        if not user_id:
+            return jsonify({"error": "Authentication required"}), 401
+        user = await get_user_by_id(user_id)
+        if not user:
+            session.clear()
+            return jsonify({"error": "Authentication required"}), 401
+        if user.role != "admin":
+            return jsonify({"error": "Admin access required"}), 403
+        g.user = user
+        return await f(*args, **kwargs)
+    return decorated
+
+
+def get_current_user_id() -> int:
+    return g.user.id