feat(memory): complete memory service with passing tests

- model: Memory struct with categories/decay/keywords
- repository: PG CRUD + keyword search + table migration SQL
- service: dedup (Jaccard), keyword extraction, decay formula
- 7 tests (3 DB integration + 4 unit), 96 total, vet clean

backend/services/memory/repository.go

@@ -36,6 +36,9 @@ func (r *Repository) Create(ctx context.Context, m *Memory) (string, error) {
 	if m.Decay == 0 {
 		m.Decay = 1.0
 	}
+	if m.Keywords == nil {
+		m.Keywords = []string{}
+	}
 
 	var id string
 	err := r.db.QueryRowContext(ctx, query,