fix: INSIGHTFACE_HOME path, entrypoint, caching default, CRON_SCHEDULE opt-in

- Dockerfile: ENV INSIGHTFACE_HOME=/models → /models/.insightface to
  match compose.yml and .env.example; the old value caused InsightFace
  to store models at /models/models/buffalo_l (double-appended subdir)
- entrypoint.sh: use /app/.venv/bin/winnow (installed entry point)
  instead of python -m winnow.cli
- config.py: ENABLE_CACHE default false → true; embedding cache is
  always beneficial in practice; users can opt out with ENABLE_CACHE=false
- compose.yml: comment out CRON_SCHEDULE so scheduling is opt-in;
  flip ENABLE_CACHE to commented opt-out to reflect new default
- README.md: update ENABLE_CACHE default documentation to true
- tests/test_config.py: update default assertion to match

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

compose.yml

@@ -35,7 +35,7 @@ services:
 
       # ── Caching & Models ──────────────────────────────────────────────────
       - FORCE_CPU=false
-      - ENABLE_CACHE=true
+      # - ENABLE_CACHE=false          # Disable embedding cache (default: true)
       - CACHE_DIR=/app/.if_cache
       - HF_HOME=/models/huggingface
       - INSIGHTFACE_HOME=/models/.insightface
@@ -47,10 +47,9 @@ services:
 
       # ── Scheduling ────────────────────────────────────────────────────────
       # Cron expression (unset = run once and exit)
-      # Every Sunday at 3 AM:
-      - CRON_SCHEDULE=0 3 * * 0
-      # - CRON_SCHEDULE=0 3 1 * *
-      # - CRON_SCHEDULE=*/30 * * * *
+      # - CRON_SCHEDULE=0 3 * * 0    # Every Sunday at 3 AM
+      # - CRON_SCHEDULE=0 3 1 * *    # First of every month
+      # - CRON_SCHEDULE=*/30 * * * * # Every 30 minutes
     volumes:
       # Replace with absolute paths on your host, e.g. /opt/winnow/models
       - /path/to/winnow/models:/models