Performance Optimierungen: Rate Limit, created_at Index, Keyset-Pagination

1. Rate Limit erhoeht: 60 -> 300 Requests/Minute (fuer 100+ User)
2. Migration 0099: created_at DESC Index auf allen Tabellen (Order by Performance)
3. Keyset-Pagination: optionaler cursor Parameter fuer contacts API
   - cursor=UUID nutzt WHERE id > cursor statt OFFSET
   - Backward compatible: ohne cursor wird page/page_size genutzt
   - next_cursor in Response fuer naechste Seite

Tests: 43/43 bestanden

app/services/contact_service.py

@@ -147,10 +147,16 @@ async def list_contacts(
     resolved_perms: dict | None = None,
     user_id: uuid.UUID | None = None,
     is_system_admin: bool = False,
+    cursor: str | None = None,
 ) -> dict:
     """List contacts with pagination, FTS search, type/folder filter, sorting.
 
     Applies row-level visibility filter based on ownership and entity_permissions.
+
+    Keyset-Pagination: If ``cursor`` is provided (a contact UUID), results are
+    filtered to ``id > cursor`` instead of using OFFSET.  This is much faster
+    for large datasets.  When ``cursor`` is not provided, classic page/page_size
+    offset pagination is used (backward compatible).
     """
     from app.core.visibility import apply_visibility_filter
 
@@ -176,6 +182,11 @@ async def list_contacts(
             Contact.search_tsv.op("@@")(func.plainto_tsquery("german", search))
         )
 
+    # Keyset-Pagination: filter by cursor if provided
+    use_keyset = cursor is not None and sort_by == "id" and sort_order == "asc"
+    if use_keyset:
+        base = base.where(Contact.id > uuid.UUID(cursor))
+
     # Count
     count_q = select(func.count()).select_from(base.subquery())
     total = (await db.execute(count_q)).scalar() or 0
@@ -187,8 +198,11 @@ async def list_contacts(
     base = base.order_by(sort_col)
 
     # Paginate
-    offset = (page - 1) * page_size
-    base = base.offset(offset).limit(page_size)
+    if use_keyset:
+        base = base.limit(page_size)
+    else:
+        offset = (page - 1) * page_size
+        base = base.offset(offset).limit(page_size)
 
     # Eager load contact_persons to avoid N+1 queries
     base = base.options(selectinload(Contact.contact_persons))
@@ -196,11 +210,17 @@ async def list_contacts(
     result = await db.execute(base)
     contacts = result.scalars().all()
 
+    # Next cursor for keyset pagination
+    next_cursor = None
+    if use_keyset and len(contacts) == page_size and contacts:
+        next_cursor = str(contacts[-1].id)
+
     return {
         "items": [_serialize_contact(c) for c in contacts],
         "total": total,
         "page": page,
         "page_size": page_size,
+        "next_cursor": next_cursor,
     }