feat: 15-minute rental slots + compact calendar cell display

- Rental bookings now use minutes since midnight (start_hour/end_hour
  store e.g. 495 for 8:15, 570 for 9:30) — DB migration 017
- Buffer applied in exact minutes (no hour ceiling): 15min buffer after
  9:00 booking → next slot available at 9:15
- Calendar cell: removed working-hours line (8:00–22:00), now shows
  colored dot + compact time per booking (8–10, 8:15–9:15)
- Time selects show HH:MM format for 15-min intervals

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

backend/migrations/017_rental_minutes.sql

@@ -0,0 +1,6 @@
+-- Convert rental_bookings start_hour/end_hour from whole hours to minutes since midnight
+-- Safe: only converts rows where start_hour < 25 (i.e., stored as hours, not minutes yet)
+UPDATE rental_bookings
+SET start_hour = start_hour * 60,
+    end_hour   = end_hour   * 60
+WHERE start_hour < 25;