Schedule backups in local time instead of UTC (0.66.0)

Schedule times were UTC, so a nightly 03:00 backup had to be entered as
01:00 or 02:00 depending on the season, and moved by an hour twice a year.

Each schedule now has an IANA time zone, preset in the form to the
browser's (e.g. Europe/Berlin), and its hour/minute/weekday are wall-clock
time there. compute_next_run builds each candidate day from its date in
that zone rather than adding 24 hours, so the time stays put across
daylight saving changes; a time skipped by the spring change runs an hour
later that day. next_run is still stored as naive UTC.

- Existing schedules default to UTC and keep their run times until edited.
- The API validates the zone and hour/minute/weekday ranges (400) instead
  of silently wrapping them.
- tzdata is a dependency: the slim base image has no system tz database.

Tests cover Berlin winter and summer time, both 2026 change days, the
skipped hour, a weekly run across the UTC date line, hourly at a +5:30
offset, the UTC fallback and the API checks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -644,8 +644,12 @@ it is what your saved destination credentials are encrypted with.
 ### Phase 7 — Scheduled backups
 
 - **Recurring backups**: schedule stacks to back up to a destination **hourly,
-  daily, or weekly** (UTC) under **Settings → Scheduled backups**. A background
+  daily, or weekly** under **Settings → Scheduled backups**. A background
   scheduler runs due jobs every minute and records last/next run + status.
+- **Local time** (0.66.0): each schedule has a time zone, preset to the
+  browser's (e.g. `Europe/Berlin`), and its time is wall-clock time there — a
+  03:00 backup stays at 03:00 across daylight saving changes. Schedules made
+  before were in UTC and stay so until edited.
 - **Several stacks per schedule** (0.65.0): tick any number of stacks, or
   *All stacks* to include stacks added later without editing the schedule. They
   are backed up one after another, each to its own file with its own retention