Fire accepted routines every interval, not once (Vikunja #366)

The tick loop now reads accepted routines from the store and nudges when
their interval has passed; accepting no longer builds a one-shot reminder.
Look at routine.DueAccepted for the schedule rule (no catch-up backlog) and
at fireAcceptedRoutines for the restraint gate — routines do not bypass it.

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

internal/routine/routine_test.go

@@ -5,6 +5,40 @@ import (
 	"time"
 )
 
+func TestDueAcceptedFiresOncePerInterval(t *testing.T) {
+	accepted := time.Date(2026, 7, 1, 9, 0, 0, 0, time.UTC)
+	fired := accepted.Add(3 * 24 * time.Hour)
+	rs := []Accepted{
+		{ID: 1, Name: "полить цветы", IntervalDays: 3, Accepted: accepted},
+		{ID: 2, Name: "покормить рыб", IntervalDays: 3, Accepted: accepted, LastFired: &fired},
+		{ID: 3, Name: "битый интервал", IntervalDays: 0, Accepted: accepted},
+	}
+
+	// One day in: nothing has waited a full interval.
+	if got := DueAccepted(rs, accepted.Add(24*time.Hour)); len(got) != 0 {
+		t.Fatalf("want nothing due after 1 day, got %+v", got)
+	}
+
+	// Three days in: the never-fired one is due. The one that already fired at
+	// day 3 starts its next three days from there. A zero interval never fires.
+	got := DueAccepted(rs, fired)
+	if len(got) != 1 || got[0].ID != 1 {
+		t.Fatalf("want only routine 1 due at day 3, got %+v", got)
+	}
+
+	// Six days in: both real routines are due.
+	if got := DueAccepted(rs, accepted.Add(6*24*time.Hour)); len(got) != 2 {
+		t.Fatalf("want both routines due at day 6, got %+v", got)
+	}
+
+	// A month later the zero-interval routine is still silent.
+	for _, r := range DueAccepted(rs, accepted.Add(30*24*time.Hour)) {
+		if r.ID == 3 {
+			t.Fatal("a routine with a zero interval must never fire")
+		}
+	}
+}
+
 func TestValidate(t *testing.T) {
 	ok := []Routine{{Name: "morning", Cron: "0 8 * * *", Body: "доброе утро"}}
 	if err := Validate(ok); err != nil {