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/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",
+    )