refactor: rename CACHE_DIR → DATA_DIR, container path .if_cache → data (#21)

* refactor: rename CACHE_DIR to DATA_DIR, default path .if_cache → data

CACHE_DIR held both the embedding cache and the SQLite tracker DB, making
the name misleading. DATA_DIR is more accurate.

- Config reads DATA_DIR first; falls back to CACHE_DIR with a deprecation
  warning so existing setups don't break on upgrade
- Default local path: data (was .if_cache)
- Docker default path: /app/data (was /app/.if_cache)
- Internal references (embeddings.py, upload_tracker.py) updated to DATA_DIR
- compose.yml, .env.example, README, wiki, and changelog updated
- Version bumped to 0.5.1

* chore: update lockfile

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.0"
+version = "0.5.1"
 description = "Selects diverse, high-quality photos from Immich as training data for Frigate face recognition."
 license = "AGPL-3.0-or-later"
 requires-python = ">=3.13"