Add the scheduler core with period keys and catch-up

internal/scheduler runs registered schedules (a name, a daily or weekly
rule read from the household settings, and a run function) in the
household zone with an injectable clock. Each fire time's period is
claimed by inserting schedule_runs(schedule, period_key) with ON
CONFLICT DO NOTHING, so a period runs at most once across restarts and
instances. At boot only each schedule's most recent fire time is
considered: within 24 h it runs once, older it is claimed as missed so
it never runs later. Settings changes move the next fire time through
SettingsService.Subscribe. DST follows household.LocalTime. The server
starts it with the worker context and waits for it before closing the
database; the concrete schedules come in later Feature 20 tasks.

Maestro task f7024755-c3d3-46f3-b798-e242ad676c1e.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -785,6 +785,20 @@ purge are fixed and not configurable.
 
 Runs are recorded in `schedule_runs` and executed through `jobs`.
 
+`internal/scheduler` runs them. A schedule registers a name, a rule read
+from the settings (`scheduler.Daily`, `Weekly` or `WeeklyForNextWeek`) and a
+run function; settings changes reach it through `SettingsService.Subscribe`
+and move the next fire time without a restart. Each fire time has a period
+key (`Rule.PeriodKey`): the local date for a daily rule, the ISO week of the
+fire date for a weekly one, or of the week after for `WeeklyForNextWeek`
+(`plan_draft`). Inserting `schedule_runs(schedule, period_key)` is the claim;
+a period already claimed is skipped, so a period runs at most once and a
+failed run is not retried. At boot only each schedule's most recent fire
+time is considered: it runs if it passed within 24 hours, and is otherwise
+claimed without running (logged as missed) so it never runs later. DST
+follows `household.LocalTime`: a skipped time fires later by the gap, a
+repeated time fires once, at its first occurrence.
+
 ## Notifications
 
 - Events: `plan_ready`, `list_ready`, `day_of_reminder`, `new_suggestions`,