feat(worker): Similar photos join bursts — OpenCV feature matching on thumbnails #298

Parent

#296

What to build

A retake or slightly panned frame from the same camera joins a neighbour's burst: two photos are similar when OpenCV feature matching (ORB keypoints, RANSAC for one consistent transform) on their stored 300 px thumbnails reaches a configurable share of matching keypoints. Matching chains, is bounded in time, and connects camera-labelled bursts.

Acceptance criteria

• A picture-comparison seam (two thumbnails in, a similarity score 0–1 out) with an OpenCV implementation (OpenCvSharp or Emgu CV), after confirming a native linux-arm64 build exists and the licence is compatible with AGPL-3.0. If neither qualifies, stop and bring it back to the maintainer. The Worker image runs it on amd64 and arm64.
• An ADR records the OpenCV dependency and why perceptual hashes were rejected (measurements in spec: Bursts — group photos of the same scene by camera label or feature similarity, show one cover per burst #296).
• Outside a BurstUUID, a photo joins only when it matches a neighbouring frame; time alone never groups.
• Matching chains (AB, BC puts all three in one burst), with consecutive frames at most 2 minutes apart and a whole burst spanning at most 5 minutes.
• Only photos with an EXIF capture time are compared, and only with frames of the same camera make and model; two photos without camera data count as different cameras.
• A similar photo joins a BurstUUID burst; a photo matching frames of two bursts merges them, the stored cover being the earliest member.
• Unchecked photos are compared against their time neighbours, checked or not; checked pairs are not compared again.
• Settings Worker:Burst… for gap (2 min), span (5 min) and similarity threshold (about 25%), Worker-wide, validated at startup.
• Each compared pair's score is logged at Debug; the run's result adds pairs matched and not matched.
• The real comparison is tested on committed fixtures: a 300 px copy of the maintainer's IMG_4386/4387 without EXIF (approved) scores above the default threshold, clearly different images below it. Grouping rules are tested at the handler seam with a substituted comparison. Mutation score 100% on the changed files.
• Repair deletes bursts with no member rows in one statement (ExecuteDelete), as the design note's section 5 step 1 says, instead of loading and EF-removing them in every config's run. Today two per-user Workers repairing at once both remove the same memberless burst, and the second SaveChanges throws DbUpdateConcurrencyException and fails that BurstDetection item. Found in the 2026-10-24 architecture review (Architecture review due (10 PRs since arch-review/2026-10-18) #302).

First caller

The BurstDetection handler from #297 calls the picture comparison for each candidate pair.

Design

Design note: #296 (comment) (sections 4, 5)
Decision (maintainer, 2026-10-23): #296 (comment) — OpenCvSharp4 (4A); the smoke test in the Worker image on amd64 and arm64 comes first, and the ADR is written here.