Multi-user sharing, groups, and in-app notifications

Backend:
- Alembic migration 0025: groups, group_memberships, project_shares,
  note_shares, notifications tables
- models: Group, GroupMembership, ProjectShare, NoteShare, Notification
- services/access.py: permission resolution (viewer/editor/admin/owner)
- services/groups.py + routes/groups.py: full group CRUD + membership
- services/sharing.py + routes/shares.py: project/note sharing API
- services/notifications.py: in-app notification create/list/mark-read
- routes/in_app_notifications.py: GET/POST notification endpoints
- routes/users.py: user search endpoint
- services/projects.py + services/notes.py: *_for_user variants
- routes updated to use *_for_user on get/list; adds permission field
- app.py: register all new blueprints

Frontend:
- api/client.ts: ShareEntry, GroupEntry, NotificationEntry types + helpers
- stores/notifications.ts: Pinia store with polling
- NotificationBell.vue: bell icon + badge + dropdown toggle + 60s poll
- NotificationsPanel.vue: unread notification list with mark-all-read
- ShareDialog.vue: teleport modal for sharing projects/notes with users/groups
- SharedWithMeView.vue: /shared route listing shared projects and notes
- AppHeader: NotificationBell, Shared nav link
- ProjectView, NoteViewerView, TaskViewerView: Share button + ShareDialog
- SettingsView: Groups admin tab with create/delete groups + member mgmt
- router: /shared route

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/models/notification.py

@@ -0,0 +1,31 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Integer, Text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from fabledassistant.models import Base
+from fabledassistant.models.base import CreatedAtMixin
+
+
+class Notification(Base, CreatedAtMixin):
+    __tablename__ = "notifications"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True)
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    # project_shared | note_shared | group_added
+    type: Mapped[str] = mapped_column(Text, nullable=False)
+    payload: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
+    read_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    def to_dict(self) -> dict:
+        return {
+            "id": self.id,
+            "user_id": self.user_id,
+            "type": self.type,
+            "payload": self.payload,
+            "read_at": self.read_at.isoformat() if self.read_at else None,
+            "created_at": self.created_at.isoformat(),
+        }