fix(index): tolerate orphan shots in reset-search-index

[nledez]

Problem

• zou reset-search-index crashes with SequenceNotFoundException on some instances that contain orphan shots (their parent_id points to a sequence that no longer exists). The exception bubbles up to the CLI, the shot index is never rebuilt, and everything after it is skipped.

Solution

• prepare_shot (zou/app/services/index_service.py) now catches SequenceNotFoundException and EpisodeNotFoundException. When the parent sequence or episode is missing, the shot is still indexed with its id, name, and project_id — keeping it discoverable in Kitsu so users can spot and fix the DB inconsistency.
• Initialize sequence = None to close the NameError path.
• Add a test (tests/services/test_index_service.py::test_reset_index_with_orphan_shot) that mocks get_sequence to simulate the orphan case and verifies that reset_index() completes without error.