Add accepted-routine timestamps to the store (Vikunja #366)

Data layer only. Migration #8 adds accepted_ts and last_fired_ts to
proposed_routines, plus ListAcceptedRoutines and MarkRoutineFired so the
tick loop can own the schedule. Accepting no longer links a reminder id.
Look at the TODO(vikunja#366) in cmd/mavend/tick.go for the next commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

cmd/mavend/tick.go

@@ -186,6 +186,10 @@ func (t *tickLoop) tick(ctx context.Context, now time.Time) {
 	// LLM-phrased — so a routine can't hallucinate. severity comes from config.
 	t.fireRoutines(ctx, now, state)
 
+	// TODO(vikunja#366): fire accepted routines here — read
+	// store.ListAcceptedRoutines, pick the ones whose interval has passed, nudge
+	// them through the gate, then MarkRoutineFired.
+
 	// morning routines: daily checklists (medicine/water/pets/...), nagged at
 	// most once per day per routine, and only for items still unevidenced at
 	// nudge time. See internal/morning for the "why not four timers" rationale.