Phase 4: backups w/ volumes, notifications, settings & users, audit page (0.4.0)

- Backup/restore: per-stack tar.gz incl. named-volume snapshots (helper
  container), upload restore with rename/overwrite/conflict detection.
- Notifications: ntfy/Discord/Slack/Gotify/generic webhooks, per-event
  subscriptions; wired into the update checker and stack lifecycle.
- Settings page: update-check interval, webhook CRUD + test, user management
  (with last-admin safeguards).
- Audit log page (searchable, paginated).
- Mobile-responsive sidebar/layout.

Multi-host agents and remote backup destinations (SFTP/S3) deferred.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/models/setting.py

@@ -0,0 +1,85 @@
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlmodel import Field, SQLModel
+
+
+def _now() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+# Notification event names ----------------------------------------------------
+EVENT_UPDATE_AVAILABLE = "update_available"
+EVENT_STACK_START = "stack_start"
+EVENT_STACK_STOP = "stack_stop"
+EVENT_STACK_ERROR = "stack_error"
+EVENT_PULL_FAILED = "pull_failed"
+
+ALL_EVENTS = [
+    EVENT_UPDATE_AVAILABLE,
+    EVENT_STACK_START,
+    EVENT_STACK_STOP,
+    EVENT_STACK_ERROR,
+    EVENT_PULL_FAILED,
+]
+
+WEBHOOK_TYPES = ["ntfy", "discord", "slack", "gotify", "generic"]
+
+
+class Setting(SQLModel, table=True):
+    """Simple key/value store for runtime-tunable settings (JSON-encoded value)."""
+
+    key: str = Field(primary_key=True)
+    value: str  # JSON-encoded
+
+
+class Webhook(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str
+    url: str
+    type: str = "generic"  # one of WEBHOOK_TYPES
+    events: str = ",".join(ALL_EVENTS)  # comma-separated subscribed events
+    enabled: bool = True
+    created_at: datetime = Field(default_factory=_now)
+
+
+# --- API schemas ---
+
+
+class WebhookCreate(SQLModel):
+    name: str
+    url: str
+    type: str = "generic"
+    events: list[str] = ALL_EVENTS
+    enabled: bool = True
+
+
+class WebhookUpdate(SQLModel):
+    name: Optional[str] = None
+    url: Optional[str] = None
+    type: Optional[str] = None
+    events: Optional[list[str]] = None
+    enabled: Optional[bool] = None
+
+
+class WebhookRead(SQLModel):
+    id: int
+    name: str
+    url: str
+    type: str
+    events: list[str]
+    enabled: bool
+    created_at: datetime
+
+
+class SettingsRead(SQLModel):
+    update_check_interval_minutes: int
+    env_webhook_count: int
+    available_events: list[str]
+    webhook_types: list[str]
+
+
+class SettingsUpdate(SQLModel):
+    update_check_interval_minutes: Optional[int] = None