BE-CAL-API-007: implement date-list API endpoint

- Add GET /calendar/dates endpoint that returns sorted future dates
  with at least one materialized (real) slot
- Excludes skipped/aborted slots and pure plan-generated virtual dates
- Add DateListResponse schema

app/api/routers/calendar.py

@@ -7,6 +7,7 @@ BE-CAL-API-003: Calendar slot edit endpoints (real + virtual).
 BE-CAL-API-004: Calendar slot cancel endpoints (real + virtual).
 BE-CAL-API-005: Plan schedule / plan list endpoints.
 BE-CAL-API-006: Plan edit / plan cancel endpoints.
+BE-CAL-API-007: Date-list endpoint.
 """
 
 from datetime import date as date_type
@@ -22,6 +23,7 @@ from app.models.models import User
 from app.schemas.calendar import (
     CalendarDayResponse,
     CalendarSlotItem,
+    DateListResponse,
     MinimumWorkloadConfig,
     MinimumWorkloadResponse,
     MinimumWorkloadUpdate,
@@ -841,6 +843,49 @@ def cancel_plan(
     )
 
 
+# ---------------------------------------------------------------------------
+# Date list (BE-CAL-API-007)
+# ---------------------------------------------------------------------------
+
+# Statuses considered inactive — slots with these statuses are excluded
+# from the date-list result because they no longer occupy calendar time.
+_DATE_LIST_EXCLUDED_STATUSES = {SlotStatus.SKIPPED.value, SlotStatus.ABORTED.value}
+
+
+@router.get(
+    "/dates",
+    response_model=DateListResponse,
+    summary="List future dates that have materialized slots",
+)
+def list_dates(
+    db: Session = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+):
+    """Return a sorted list of future dates that have at least one
+    materialized (real) slot.
+
+    - Only dates **today or later** are included.
+    - Only **active** slots are counted (skipped / aborted are excluded).
+    - Pure plan-generated (virtual) dates that have not been materialized
+      are **not** included.
+    """
+    today = date_type.today()
+
+    rows = (
+        db.query(TimeSlot.date)
+        .filter(
+            TimeSlot.user_id == current_user.id,
+            TimeSlot.date >= today,
+            TimeSlot.status.notin_(list(_DATE_LIST_EXCLUDED_STATUSES)),
+        )
+        .group_by(TimeSlot.date)
+        .order_by(TimeSlot.date.asc())
+        .all()
+    )
+
+    return DateListResponse(dates=[r[0] for r in rows])
+
+
 # ---------------------------------------------------------------------------
 # MinimumWorkload
 # ---------------------------------------------------------------------------

app/schemas/calendar.py

@@ -391,3 +391,19 @@ class SchedulePlanCancelResponse(BaseModel):
         default_factory=list,
         description="IDs of past materialized slots that were NOT affected",
     )
+
+
+# ---------------------------------------------------------------------------
+# Calendar date-list (BE-CAL-API-007)
+# ---------------------------------------------------------------------------
+
+class DateListResponse(BaseModel):
+    """Response for the date-list endpoint.
+
+    Returns only dates that have at least one materialized (real) future
+    slot.  Pure plan-generated (virtual) dates are excluded.
+    """
+    dates: list[date] = Field(
+        default_factory=list,
+        description="Sorted list of future dates with materialized slots",
+    )