refactor: rename CACHE_DIR → DATA_DIR, container path .if_cache → data (#21) (#22)

* 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>

.env.example

@@ -34,7 +34,7 @@ STRATEGY=adaptive
 # ── Caching & Models ──────────────────────────────────────────────────────────
 # FORCE_CPU=true              # Disable GPU, fall back to CPU
 # ENABLE_CACHE=false          # Disable embedding cache (default: true)
-CACHE_DIR=/app/.if_cache
+DATA_DIR=/app/data
 INSIGHTFACE_HOME=/models/.insightface
 
 # ── Tracker overrides (one-shot — remove after use) ───────────────────────────