Add opt-in hash DB batching and atomic JSON writes

[TerryFrench]

Summary

Adds opt-in hash DB flush batching for import and makes local JSON DB writes atomic.

Changes

• Adds --hash-db-batch-size to import (default 1, preserving current behavior).
• Enables periodic hash DB flush when batching is enabled.
• Makes hash.json / location.json writes atomic (tempfile + os.replace).
• Adds tests for:
  • batch flush behavior
  • atomic replace write behavior
  • process-file no-flush path when batching

Why

Improves resilience and performance tradeoff options for large imports while keeping default behavior unchanged.

Dependency

This PR is based on #513 (pr/06-interrupt-exiftool) because it builds on nearby CLI/filesystem changes.

Tests

Validated locally with pytest after applying this change.

Targeted coverage added for new behavior

• elodie/tests/elodie_test.py::test_import_hash_db_batch_size_default_writes_per_file
  • verifies default behavior remains unchanged (--hash-db-batch-size=1, per-file write path).
• elodie/tests/elodie_test.py::test_import_hash_db_batch_size_flushes_in_batches
  • verifies opt-in batching flushes hash DB at the expected interval.
• elodie/tests/filesystem_test.py::test_process_file_skips_hash_db_flush_when_write_db_false
  • verifies process_file skips immediate hash DB flush when batching mode is active.

Suite run

• Ran project test suite
• Result: all selected tests passed (no regressions in modified areas).