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/config.py

@@ -1,11 +1,28 @@
 import os
 
 
+def _read_secret(env_var: str, secret_file_var: str, default: str) -> str:
+    """Read a config value from env var, or from a Docker secrets file."""
+    value = os.environ.get(env_var)
+    if value:
+        return value
+    secret_file = os.environ.get(secret_file_var)
+    if secret_file:
+        try:
+            with open(secret_file) as f:
+                return f.read().strip()
+        except FileNotFoundError:
+            pass
+    return default
+
+
 class Config:
-    DATABASE_URL: str = os.environ.get(
+    DATABASE_URL: str = _read_secret(
         "DATABASE_URL",
+        "DATABASE_URL_FILE",
         "postgresql+asyncpg://fabled:fabled@localhost:5432/fabledassistant",
     )
     OLLAMA_URL: str = os.environ.get("OLLAMA_URL", "http://localhost:11434")
     OLLAMA_MODEL: str = os.environ.get("OLLAMA_MODEL", "llama3.1")
-    SECRET_KEY: str = os.environ.get("SECRET_KEY", "dev-secret-change-me")
+    SECRET_KEY: str = _read_secret("SECRET_KEY", "SECRET_KEY_FILE", "dev-secret-change-me")
+    LOG_LEVEL: str = os.environ.get("LOG_LEVEL", "INFO")