feat: Aufgaben (Tasks) mit CalDAV VTODO-Sync

Neuer Menuepunkt "Aufgaben" unterhalb Kontakte.

Backend:
- TaskList + Task + TaskListShare Models
- REST-API: CRUD, Teilen, my-color, Import/Export (.ics mit VTODO, CSV)
- CalDAV: Task-Listen tauchen als Calendar-Collection mit
  supported-calendar-component-set=VTODO im autodiscovery auf
- PROPFIND/REPORT/GET/PUT/DELETE/PROPPATCH/MKCOL fuer /dav/<user>/tl-<id>/
- SSE-Notifications bei Aenderungen

Frontend:
- TasksView mit Listen-Sidebar, Suche, "Erledigte ausblenden"
- Mehrfachauswahl + Bulk-Loeschen, Status-Toggle per Checkbox
- Editor mit Titel/Beschreibung/Faellig/Prioritaet/Status/Fortschritt
- Teilen, Farbe persoenlich anpassen, Import/Export

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/models/task.py

@@ -0,0 +1,86 @@
+from datetime import datetime, timezone
+
+from app.extensions import db
+
+
+class TaskList(db.Model):
+    __tablename__ = 'task_lists'
+
+    id = db.Column(db.Integer, primary_key=True)
+    owner_id = db.Column(db.Integer, db.ForeignKey('users.id'), nullable=False, index=True)
+    name = db.Column(db.String(255), nullable=False)
+    color = db.Column(db.String(7), default='#10b981')
+    description = db.Column(db.Text, nullable=True)
+    created_at = db.Column(db.DateTime, default=lambda: datetime.now(timezone.utc))
+    updated_at = db.Column(db.DateTime, default=lambda: datetime.now(timezone.utc),
+                           onupdate=lambda: datetime.now(timezone.utc))
+
+    tasks = db.relationship('Task', backref='task_list', lazy='dynamic',
+                            cascade='all, delete-orphan')
+    shares = db.relationship('TaskListShare', backref='task_list', lazy='dynamic',
+                             cascade='all, delete-orphan')
+
+    def to_dict(self):
+        return {
+            'id': self.id,
+            'owner_id': self.owner_id,
+            'name': self.name,
+            'color': self.color,
+            'description': self.description,
+            'created_at': self.created_at.isoformat() if self.created_at else None,
+        }
+
+
+class Task(db.Model):
+    __tablename__ = 'tasks'
+
+    id = db.Column(db.Integer, primary_key=True)
+    task_list_id = db.Column(db.Integer, db.ForeignKey('task_lists.id'), nullable=False, index=True)
+    uid = db.Column(db.String(255), unique=True, nullable=False)
+    ical_data = db.Column(db.Text, nullable=False, default='')  # Full VTODO block
+    summary = db.Column(db.String(500), nullable=True)
+    description = db.Column(db.Text, nullable=True)
+    status = db.Column(db.String(32), nullable=True)  # NEEDS-ACTION | IN-PROCESS | COMPLETED | CANCELLED
+    priority = db.Column(db.Integer, nullable=True)  # 0 (keine) - 9
+    percent_complete = db.Column(db.Integer, nullable=True)  # 0..100
+    due = db.Column(db.DateTime, nullable=True, index=True)
+    dtstart = db.Column(db.DateTime, nullable=True)
+    completed_at = db.Column(db.DateTime, nullable=True)
+    categories = db.Column(db.Text, nullable=True)  # kommagetrennt
+    created_at = db.Column(db.DateTime, default=lambda: datetime.now(timezone.utc))
+    updated_at = db.Column(db.DateTime, default=lambda: datetime.now(timezone.utc),
+                           onupdate=lambda: datetime.now(timezone.utc))
+
+    def to_dict(self):
+        return {
+            'id': self.id,
+            'task_list_id': self.task_list_id,
+            'uid': self.uid,
+            'summary': self.summary,
+            'description': self.description,
+            'status': self.status or 'NEEDS-ACTION',
+            'priority': self.priority,
+            'percent_complete': self.percent_complete,
+            'due': self.due.isoformat() if self.due else None,
+            'dtstart': self.dtstart.isoformat() if self.dtstart else None,
+            'completed_at': self.completed_at.isoformat() if self.completed_at else None,
+            'categories': self.categories.split(',') if self.categories else [],
+            'created_at': self.created_at.isoformat() if self.created_at else None,
+            'updated_at': self.updated_at.isoformat() if self.updated_at else None,
+        }
+
+
+class TaskListShare(db.Model):
+    __tablename__ = 'task_list_shares'
+
+    id = db.Column(db.Integer, primary_key=True)
+    task_list_id = db.Column(db.Integer, db.ForeignKey('task_lists.id'), nullable=False, index=True)
+    shared_with_id = db.Column(db.Integer, db.ForeignKey('users.id'), nullable=False, index=True)
+    permission = db.Column(db.String(20), nullable=False, default='read')
+    color = db.Column(db.String(7), nullable=True)
+
+    shared_with = db.relationship('User', backref='shared_task_lists')
+
+    __table_args__ = (
+        db.UniqueConstraint('task_list_id', 'shared_with_id', name='uq_task_list_share'),
+    )