feat: notification preferences system with plugin registry

- Add NotificationType and NotificationPreference models
- Add get_notification_types() to BasePlugin for plugin registration
- Add sync_notification_types() to PluginRegistry
- Update create_notification() to check user preferences (returns Notification|None)
- Add API endpoints: GET /types, GET /preferences, PATCH /preferences/{type_key}
- Add NotificationPreferenceUpdate schema
- Mail plugin registers 10 notification types with metadata
- Add Alembic migration 0017 for new tables + seed data
- Frontend: SettingsNotifications page with toggle switches
- Frontend: Settings tab, route, hooks for notification preferences
- i18n: notification settings keys (de/en)

app/models/notification.py

@@ -1,11 +1,20 @@
-"""Notification model."""
+"""Notification models."""
 
 from __future__ import annotations
 
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Index, String, Text, func
+from sqlalchemy import (
+    Boolean,
+    DateTime,
+    ForeignKey,
+    Index,
+    String,
+    Text,
+    UniqueConstraint,
+    func,
+)
 from sqlalchemy.dialects.postgresql import UUID as PGUUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -33,3 +42,49 @@ class Notification(Base, TenantMixin):
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
+
+
+class NotificationType(Base):
+    """Registered notification type from a plugin."""
+
+    __tablename__ = "notification_types"
+    __table_args__ = (Index("ix_notification_types_key", "type_key"),)
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    type_key: Mapped[str] = mapped_column(String(20), nullable=False, unique=True)
+    plugin_name: Mapped[str] = mapped_column(String(100), nullable=False)
+    category: Mapped[str] = mapped_column(String(50), nullable=False, default="general")
+    label: Mapped[str] = mapped_column(String(200), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    is_enabled_by_default: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+
+class NotificationPreference(Base, TenantMixin):
+    """User preference for a notification type (opt-in/opt-out)."""
+
+    __tablename__ = "notification_preferences"
+    __table_args__ = (
+        UniqueConstraint("user_id", "type_key", name="uq_notif_pref_user_type"),
+        Index("ix_notif_prefs_user", "user_id"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        PGUUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    type_key: Mapped[str] = mapped_column(String(20), nullable=False)
+    is_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )