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>

tests/test_upload_tracker.py

@@ -7,7 +7,7 @@ import pytest
 @pytest.fixture(autouse=True)
 def isolated_cache(monkeypatch, tmp_path):
     """Point tracker at a temp directory so tests don't touch real cache files."""
-    monkeypatch.setenv("CACHE_DIR", str(tmp_path))
+    monkeypatch.setenv("DATA_DIR", str(tmp_path))
     from winnow.config import _Config
     _Config.reset()
     # Also reset the SQLite connection so the next call opens the new path