Add quality replacement for Frigate face training images

When a person is at MAX_AUTO_IMAGES, winnow now replaces the
lowest-quality mapped training image in Frigate if a higher-confidence
candidate is available, keeping the training set always optimised.

Only files winnow uploaded (tracked via frigate_files mapping) are ever
replaced — manually added Frigate training images are never touched.
A concurrent-upload race condition is detected per-file: if N>1 new
files appear after one upload, the mapping is skipped rather than
guessed, logging at INFO level. The per-file snapshot approach is
retained over a batch approach because wrong mappings (which a batch
approach risks on race) are worse than no mapping.

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

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- **Quality replacement**: when a person is at `MAX_AUTO_IMAGES`, winnow now checks each new candidate against the lowest-quality image already in Frigate and swaps it in if the new image scores higher. Only images winnow uploaded (tracked in `frigate_files`) are ever replaced — files added manually through Frigate's UI are left untouched permanently. Enabled by default; set `QUALITY_REPLACEMENT=false` to revert to the previous behaviour of skipping people at cap.
+- **Frigate filename mapping**: each successful upload now records the mapping from Frigate's assigned filename to the originating Immich asset ID and face confidence score in the tracker (`frigate_files` field). This is the foundation for quality replacement and future management of the Frigate training set.
+- **`QUALITY_REPLACEMENT` env var** (default `true`): controls whether at-cap people are eligible for quality replacement. When disabled, people at `MAX_AUTO_IMAGES` are skipped as before.
+
 ## [0.2.12] - 2026-06-13
 
 ### Added