feat(email): Slice 3 inline-media admission and tracking-pixel classification

[seonghobae]

Description

#1350 Slice 3 admission only (no OCR/VLM). This draft adds a deterministic, provider-free local admission contract so Naruon can resolve cid: images through the same message's multipart/related parts and refuse to treat a 1×1 beacon as document evidence.

Unresolved CID fails closed with unresolved_cid_reference. Remote http(s) images remain no-fetch; this slice does not add egress. Classification is the closed set tracking_pixel, unsupported_media, and document_image (screenshots/charts/scans share document_image here). Provenance returns source part index, Content-ID, SHA-256 of exact decoded source bytes, classification, and known / unknown evidence boundary.

Predecessor evidence: N/A and does not transfer. Previous head 51c003d701cce4eb0779ccbba96c499fb3e5379d was reconciled only onto stale develop@a81ac4f7cd18320ee550b4290308b57c0b33abbf. Exact new head 37b4af566d749028f7dc99545fff1db517d61d1a already contains live develop@dd8d15191338b841f9e6f3a06507c6a5643b95d0 via the existing normal merge Merge branch 'develop' into cursor/email-media-admission-slice3-c9de. No additional merge commit was created in this refresh. Predecessor-head tests, coverage, and review evidence do not transfer. #1376 already covers EmailMediaArtifact pixel-dimension extraction on a separate branch and is not copied or rewritten here. PNG IHDR / GIF logical-screen sizes are read only as the local tracking-pixel heuristic.

Refs #1350

Customer next action

After independent review, a buyer-visible mail/document pipeline can call admit_email_inline_media() before any OCR/VLM step. Send only document_image admissions downstream. Do not send tracking_pixel, unsupported_media, or unresolved_cid_reference outcomes to a model. Do not merge this draft until it is no longer Draft and a qualifying independent APPROVE exists on the unchanged head.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Exact-head verification

Previous head: 51c003d701cce4eb0779ccbba96c499fb3e5379d
Exact current head: 37b4af566d749028f7dc99545fff1db517d61d1a
Live develop SHA: dd8d15191338b841f9e6f3a06507c6a5643b95d0
Reconcile: already exact on current develop (gh confirmed baseRefOid=dd8d1519, headRefOid=37b4af56, develop is an ancestor). No new merge commit. Not a squash. Not a force-push of develop.

Focused product tests on this exact head (four fixture cases plus boundary coverage):

cd backend
PYTHONPATH=. PYTHONWARNINGS=error python3 -m pytest -c /dev/null --noconftest -p no:cacheprovider \
  tests/test_email_media_admission.py \
  tests/test_email_media_admission_boundaries.py -q
# 21 passed

Local count on 37b4af56: 21 passed. Predecessor-head counts do not transfer.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Merge boundary

This PR must remain Draft. Merge is prohibited while Draft and prohibited without a qualifying independent non-author APPROVE on the unchanged head. Do not self-approve, mark Ready, squash-merge over a draft, or treat predecessor/stacked/#1376 evidence as this head's proof.

  

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 81c1056 and 5a80583.

📒 Files selected for processing (10)

• AGENTS.md
• backend/services/email_media_admission.py
• backend/tests/fixtures/email_media_admission/cid_related_document_image.eml
• backend/tests/fixtures/email_media_admission/repeated_identical_base64.eml
• backend/tests/fixtures/email_media_admission/tracking_pixel_1x1.eml
• backend/tests/fixtures/email_media_admission/unresolved_cid.eml
• backend/tests/test_email_media_admission.py
• backend/tests/test_email_media_admission_boundaries.py
• docs/architecture/image-content-detection.md
• docs/doctoring/email-inline-media-admission.md

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[seonghobae]

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[github-actions]

PR governance metadata gate update for 5a80583bcabc22609e8677864ae86f867d85fd45: no current blocking failures remain.

PR governance metadata gate is ready; all current-head requirements passed.

[seonghobae]

@coderabbitai review

[seonghobae]

@coderabbitai review

Please review the unchanged exact current head 37b4af566d749028f7dc99545fff1db517d61d1a on live develop@dd8d1519. Predecessor 51c003d7 evidence does not transfer. Local admission tests: 21 passed. This comment is not an approval. Keep Draft.

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@seonghobae I will review exact head 37b4af566d749028f7dc99545fff1db517d61d1a against develop@dd8d1519. I will treat predecessor 51c003d7 evidence as non-transferable. The PR remains Draft.

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[seonghobae]

The CodeQL alert comment references predecessor commit 37b4af566d749028f7dc99545fff1db517d61d1a; GitHub marks alert 449 as fixed and its current instance is not open. Current head is 40aef4788971b6e7a84f36598b266b5793a7c5e7. Requested branch refresh and fresh current-head review/Checks; no source change was justified by the stale alert.

[seonghobae]

Current-head review note for 40aef47: the CodeQL alert points to an older commit where the test used an arbitrary substring check. The current head uses urlsplit(...).hostname == click.list-manage.com, and the production host matcher requires exact host or a dot-boundary suffix. CodeQL and Semgrep are successful on the current head; only the historical metadata-only gate failure remains. The protected scheduler should refresh exact-head evidence.

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

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