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/services/llm.py

@@ -149,6 +149,7 @@ def _find_urls(text: str) -> list[str]:
 
 
 async def build_context(
+    user_id: int,
     history: list[dict],
     current_note_id: int | None,
     user_message: str,
@@ -160,7 +161,7 @@ async def build_context(
     which notes were included as context.
     """
     exclude_set = set(exclude_note_ids or [])
-    assistant_name = await get_setting("assistant_name", "Fable")
+    assistant_name = await get_setting(user_id, "assistant_name", "Fable")
     system_parts = [
         f"You are a helpful assistant named {assistant_name}, integrated into a note-taking and task-tracking app called Fabled Assistant. "
         "Help users with their notes, tasks, and general questions. "
@@ -175,7 +176,7 @@ async def build_context(
 
     # Include current note context if provided
     if current_note_id:
-        note = await get_note(current_note_id)
+        note = await get_note(user_id, current_note_id)
         if note:
             context_meta["context_note_id"] = note.id
             context_meta["context_note_title"] = note.title
@@ -194,7 +195,7 @@ async def build_context(
             search_exclude.add(current_note_id)
         try:
             notes = await search_notes_for_context(
-                keywords, exclude_ids=search_exclude or None, limit=3
+                user_id, keywords, exclude_ids=search_exclude or None, limit=3
             )
             snippets: list[str] = []
             for n in notes: