chore: retire the old Docker image (plex_generate_vid_previews)

[stevezau]

Retires the old image stevezzau/plex_generate_vid_previews (renamed to stevezzau/media_preview_generator). The old name had ~57k pulls vs ~19k for the new one, so instead of mirroring forever we push a one-shot tombstone over its tags and drop the recurring deprecated build from CI.

What's here

• tombstone/ — nginx:1.27-alpine serving a "this image moved → switch to media_preview_generator" page on port 8080 (the app's port, so existing compose port-maps hit it; served for every path).
• retire-deprecated-image.yml — workflow_dispatch (gated on typing retire) that buildx-pushes the tombstone multi-arch over the old :latest + :dev. Run once after merge.
• ci.yml — deprecated image flavour removed entirely (matrix entries, outputs, manifest + digest steps, Docker Hub description step, DOCKER_IMAGE_DEPRECATED). Canonical build/merge verified intact.
• Banner + startup log — now "retired, no longer updated, switch now" (was "mirrors until 2026-10-29"); DEPRECATED_IMAGE_SUNSET_DATE removed.
• Docs — DOCKERHUB_README.md rename note updated to "retired"; deleted the orphaned DOCKERHUB_DEPRECATED_README.md.

Verification

Architecture Review run — no HIGH (canonical pipeline confirmed unbroken: no dangling docker_tags_deprecated//tmp/digests/deprecated refs, artifact↔merge pattern still digest-*-canonical). The two MED doc-drift items it found (stale README claims) are fixed in this commit. Both workflows pass YAML validation; banner tests updated + green.

Post-merge step: trigger the retire-deprecated-image workflow once to push the tombstone.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.00%. Comparing base (0269e8d) to head (f581390).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #276      +/-   ##
==========================================
- Coverage   84.03%   84.00%   -0.04%     
==========================================
  Files          89       89              
  Lines       19670    19669       -1     
==========================================
- Hits        16529    16522       -7     
- Misses       3141     3147       +6     

Files with missing lines	Coverage Δ
media_preview_generator/web/app.py	83.28% <100.00%> (ø)
media_preview_generator/web/notifications.py	92.52% <ø> (-0.07%)	⬇️

... and 2 files with indirect coverage changes

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