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/user.py

@@ -0,0 +1,32 @@
+from datetime import datetime, timezone
+
+from sqlalchemy import DateTime, Index, Text
+from sqlalchemy.orm import Mapped, mapped_column
+
+from fabledassistant.models import Base
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    username: Mapped[str] = mapped_column(Text, unique=True, nullable=False)
+    email: Mapped[str | None] = mapped_column(Text, nullable=True)
+    password_hash: Mapped[str] = mapped_column(Text, nullable=False)
+    role: Mapped[str] = mapped_column(Text, nullable=False, default="user")
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    __table_args__ = (
+        Index("ix_users_username", "username"),
+    )
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "username": self.username,
+            "email": self.email,
+            "role": self.role,
+            "created_at": self.created_at.isoformat(),
+        }