recall: a re-tapped fact drops its superseded vector (V-493)

The fact vector id carries a timestamp, so tapping the same key twice added a
row instead of replacing one and recall then scored the old value against the
current one. CorrectValue and VoidLatestFact already prune the key; an ordinary
re-tap is the third way a value is superseded and it did not.

actionFact now prunes fact:<key>: before inserting, so exactly one vector
survives per key. InMemoryStore gained the matching DeletePrefix, because a
test double that quietly kept both rows would pass a test the daemon fails.

The prune is best-effort and silent on a store that cannot do it: the fact row
is the truth, and a stale vector costs a wrong recall, not a lost fact.

internal/memory/store.go

@@ -132,6 +132,26 @@ func (s *InMemoryStore) Delete(_ context.Context, id string) error {
 	return nil
 }
 
+// DeletePrefix removes every row whose id starts with prefix and returns how
+// many went. It matches store.MemoryStore's method so a test double and the
+// real index agree about superseding a fact (Vikunja #493) — a double that
+// silently kept the old vectors would pass a test the daemon fails.
+func (s *InMemoryStore) DeletePrefix(_ context.Context, prefix string) (int64, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	kept := s.items[:0]
+	var n int64
+	for _, it := range s.items {
+		if strings.HasPrefix(it.id, prefix) {
+			n++
+			continue
+		}
+		kept = append(kept, it)
+	}
+	s.items = kept
+	return n, nil
+}
+
 func (s *InMemoryStore) Search(_ context.Context, vec []float32, topK int) ([]Result, error) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()