perf(platform): a published release reaches Flagger within a minute

[ExtraToast]

Part of #2513

Why

v1.24.1 was announced at 16:47, but Flagger only picked it up at 16:53. Every step between the two is a poll:

Step	Before	After
ImageRepository scans the registry for new tags	5 min	20s
Image automation commits the new tag	as soon as the policy changes	unchanged
The flux-system GitRepository fetches main	1 min	20s
Flux applies the change, and Flagger notices	seconds, then 10s	unchanged

The worst case falls from about 6 minutes to about 45s, and it is usually 20 to 30s.

What changes

• The four ImageRepositories (api, frontend, imaging and stalwart-tools) scan every 20s. Each gets exclusionList: ['^[^v]'], so only release tags are listed. Main pushes a sha- tag on every commit, and each scan would otherwise list them all.
• The flux-system GitRepository in gotk-sync.yaml fetches every 20s. Running flux bootstrap again would reset this value, so it has to be kept if that ever happens.

Cost

• About 12 tag listings a minute against ghcr, with the same read-only token as before. That is well under GitHub's limits.
• A git fetch from GitHub every 20s, which has no API rate limit.

Considered instead

A Flux Receiver that release.yml calls once the images are published, authenticated with GitHub's OIDC token. It would save about another 20s, but it needs a public webhook endpoint into Flux, so it was left out.

[github-actions]

Changes

Bucket	Files	Added	Removed
platform	2	███████ 16	░░░ 8
total	2	+16	−8

No production or test lines added.

Coverage

No coverage summary reached this report.

Patch coverage: this pull request changes no line that coverage measures.

No coverage reached this report for api (api-unit-test-reports).
No coverage reached this report for frontend (frontend-unit-coverage).
No coverage reached this report for imaging (imaging-unit-coverage).