Bookability: show last-updated timestamp per date column

Backend adds valid_from to DateRateInfo and computes date_last_updated dict
(max valid_from across categories per date). Frontend renders it as a small
timestamp below the date in each column header — time only if today, date+time
if older. Full ISO string in title tooltip.

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

backend/api/bookability.py

@@ -52,12 +52,14 @@ class DateRateInfo(BaseModel):
     tariffs: List[TariffInfo]
     tariff_count: int
     occupancy: Optional[OccupancyInfo] = None
+    valid_from: Optional[str] = None
 
 
 class RateMatrixResponse(BaseModel):
     categories: List[CategoryInfo]
     dates: List[str]
     matrix: Dict[str, Dict[str, DateRateInfo]]
+    date_last_updated: Dict[str, Optional[str]] = {}
 
 
 # ============================================
@@ -229,19 +231,32 @@ async def get_rate_matrix(
 
         # Preserve existing occupancy data
         existing_occ = matrix[cat_id][rate_date].occupancy
+        vf = row.valid_from.isoformat() if row.valid_from else None
 
         matrix[cat_id][rate_date] = DateRateInfo(
             rate_gross=float(row.rate_gross) if row.rate_gross else None,
             rate_net=float(row.rate_net) if row.rate_net else None,
             tariffs=tariffs_list,
             tariff_count=tariffs_data.get('tariff_count', len(tariffs_list)),
-            occupancy=existing_occ
+            occupancy=existing_occ,
+            valid_from=vf
         )
 
+    # Per-date latest update time (max valid_from across all categories for each date)
+    date_last_updated: Dict[str, Optional[str]] = {}
+    for date_str in dates:
+        latest = None
+        for cat in categories:
+            vf = matrix.get(cat.category_id, {}).get(date_str, DateRateInfo(tariffs=[], tariff_count=0)).valid_from
+            if vf and (latest is None or vf > latest):
+                latest = vf
+        date_last_updated[date_str] = latest
+
     return RateMatrixResponse(
         categories=categories,
         dates=dates,
-        matrix=matrix
+        matrix=matrix,
+        date_last_updated=date_last_updated
     )