feat(schedule_type): minute-precision windows + variable maintenance length

Lifts the two hard restrictions in PR #18:
  * window bounds were `int hour` (0-23) → now `int minutes-since-UTC-midnight` (0-1439)
  * maintenance window was exactly 1 hour → now any duration in [1, 180] minutes
    ((maint_to - maint_from) mod 1440)

## Schema migration (additive)

`_migrate_schema()` detects legacy "hours" rows (any row where MAX of the
6 window columns is ≤ 23) and multiplies each column by 60 to convert
to minutes. Idempotent — post-conversion values are well above 23 so
the guard never fires twice.

## Touched surfaces

- `models/schedule_type.py` — column comments updated; new
  `compute_maintenance_duration()` helper (returns 1-1440 min, treats
  from==to as 1440 which is then rejected by validator)
- `schemas/schedule_type.py` — `*_from`/`*_to` upper bound 23 → 1440;
  `_validate_maintenance_window` accepts 1-180min duration; response
  includes derived `maintenance_duration_minutes`
- `schemas/schedule_type_special_slot.py` — `minute_in_window` max
  59→179; `estimated_duration` max 60→180
- `routers/schedule_type.py` — PATCH re-validates merged maintenance
  pair (partial updates can put the row into an invalid combo the
  pydantic single-field validator can't catch); `_attach_derived`
  populates the new response field
- `routers/schedule_type_special_slot.py` — `_validate_fits_window`
  now takes the parent's maintenance duration instead of hard-coded 60
- `routers/calendar.py` — `_scheduled_inside_window` arg renamed
  hour→min; the maintenance-window guard error message formats
  HH:MM not HH:00
- `services/special_slot_materialiser.py` — materialised
  `scheduled_at` derived from `(maint_from_min + tpl.minute_in_window)`
  with hour/minute split

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/services/special_slot_materialiser.py

@@ -145,11 +145,11 @@ def _build_time_slot_from_template(
     schedule_type: ScheduleType,
     template: ScheduleTypeSpecialSlot,
 ) -> TimeSlot:
-    scheduled_at = time_type(
-        hour=schedule_type.maintenance_from,
-        minute=template.minute_in_window,
-        second=0,
-    )
+    # schedule_type.maintenance_from is minutes-since-UTC-midnight; the
+    # template's minute_in_window is an offset inside that window. Combined
+    # offset must fit in [0, 1440) and produce a wall-clock time_type.
+    total_min = (schedule_type.maintenance_from + template.minute_in_window) % 1440
+    scheduled_at = time_type(hour=total_min // 60, minute=total_min % 60, second=0)
     # Merge admin-supplied event_data with bookkeeping pointers so the
     # agent (and ARD) can identify the template at wake time.
     merged_event_data = dict(template.event_data or {})