* 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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