fix(db): stop writing a column that does not exist when editing an observation (#69)

UpdateObservation set updated_at_epoch, but the observations table has no such column, so every PUT /api/observations/{id} failed with HTTP 500 (no such column: updated_at_epoch). Remove the assignment; nothing reads an edited-at time. An update that changes nothing now returns the observation, as intended. UpdateObservation had no tests: add store and handler tests.

Co-authored-by: Claude Sonnet 5.5 <[email protected]>

internal/db/gorm/observation_store.go

@@ -252,9 +252,6 @@ func (s *ObservationStore) UpdateObservation(ctx context.Context, id int64, upda
 		updates["scope"] = sql.NullString{String: *update.Scope, Valid: true}
 	}
 
-	// Add updated_at timestamp
-	updates["updated_at_epoch"] = sql.NullInt64{Int64: time.Now().Unix(), Valid: true}
-
 	if len(updates) == 0 {
 		// Nothing to update, just return existing observation
 		return toModelObservation(&dbObs), nil