revert: replace SQLite tracker with JSON backend (v0.6.0) (#32)

* revert: replace SQLite tracker with JSON backend (v0.6.0)

The SQLite migration (v0.5.0) spawned 21 bug-fix releases in two days:
data-loss risk in the migration layer, schema PK conflicts on per-person
tracking, tracker isolation races under concurrent runs, and a disk-full
error that triggered duplicate Frigate uploads. The complexity cost
outweighs the benefit.

Restored the pre-SQL JSON tracker (frigate_uploaded_ids.json /
frigate_rejected_ids.json in DATA_DIR). Public API is identical — all
callers in executor.py, jobs.py, cli.py, and reconcile.py work unchanged.
Existing JSON files are read automatically; frigate_tracker.db can be
deleted once verified.

* fix: narrow corrupt-thumbnail exception to UnidentifiedImageError; restore IMMICH_URL empty-string fallback

* docs: rewrite v0.6.0 changelog, strip v0.5.x entries, fix README SQLite references

* chore: remove dead get_frigate_filename_for_asset (orphaned since FRIGATE_SCORE_THRESHOLD removal in v0.4.0)

* fix: sort imports in executor.py (ruff I001)

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.21"
+version = "0.6.0"
 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"