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/models/note.py

@@ -1,7 +1,7 @@
 import enum
 from datetime import date, datetime, timezone
 
-from sqlalchemy import Date, DateTime, Index, Text
+from sqlalchemy import Date, DateTime, ForeignKey, Index, Integer, Text
 from sqlalchemy.dialects.postgresql import ARRAY
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -25,6 +25,9 @@ class Note(Base):
     __tablename__ = "notes"
 
     id: Mapped[int] = mapped_column(primary_key=True)
+    user_id: Mapped[int | None] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=True
+    )
     title: Mapped[str] = mapped_column(Text, default="")
     body: Mapped[str] = mapped_column(Text, default="")
     tags: Mapped[list[str]] = mapped_column(ARRAY(Text), default=list)
@@ -47,6 +50,7 @@ class Note(Base):
         Index("ix_notes_tags", "tags", postgresql_using="gin"),
         Index("ix_notes_status", "status"),
         Index("ix_notes_title", "title"),
+        Index("ix_notes_user_id", "user_id"),
     )
 
     @property