Optimize import pipeline and local cache writes

[prodriguezxyz]

Summary

This PR improves import performance by reducing repeated local cache I/O and lowering ExifTool overhead during
import.

Changes

• Reused a single Db instance per command and deferred cache writes with flush().
• Ensured import and update persist successful work even when exiting with partial errors.
• Added in-memory geolocation caching plus a simple spatial index for coordinate lookups.
• Batched EXIF reads during import and limited ExifTool requests to only the tags needed for path and filename
  generation.
• Added benchmark and profiling scripts for reproducible validation.

Impact

• hash_db writes are now dramatically faster in local benchmarks.
• Geolocation lookup scales better for larger location caches.
• Profiled import on 50 plain.jpg files improved from 1.2477s to 0.9922s.

Validation

Executed:

• venv/bin/python -m pytest elodie/tests/localstorage_test.py
• venv/bin/python -m pytest elodie/tests/localstorage_test.py elodie/tests/filesystem_test.py -k
  'defers_hash_db_write'
• venv/bin/python -m pytest elodie/tests/elodie_test.py -k 'import_file_photo or
  cli_import_persists_hash_db_on_success or cli_import_flushes_successes_even_when_command_exits_with_error'