refactor(backup): share service_action_type, document vocabulary divergence

[GeiserX]

Summary

Follow-up to the review of #191 (service-action metadata in raw_data). The one genuinely actionable item from that review — pinning msg.action = None in the extended test factory — was already folded into #191 before merge, so this PR addresses the remaining polish suggestions:

• Share the helper. Move service_action_type (was the module-private _service_action_type) into src/message_utils.py, right beside extract_topic_id — the shared home both the backfill and listener already import from. Drops the now-unused re import in telegram_backup.py.
• Document the vocabulary divergence. The backfill derives action_type from low-level MessageAction class names (chat_edit_title, topic_create, …) while the live listener uses curated event names (title_changed, user_added, …). Only the raw_data shape is shared, not the values. The _process_message comment now says "structural parity (keys), not value parity," and the helper docstring spells out both this and the acronym edge.
• Document the acronym edge. re.sub(r"(?<!^)(?=[A-Z])", "_", name) splits consecutive capitals letter-by-letter, e.g. MessageActionSetMessagesTTL → set_messages_t_t_l. No title-bearing action we consume is affected, and the tag is a stable, unparsed identifier — cosmetic only, now pinned by a test so it can't silently change.
• Tests. Add a direct unit test for service_action_type (topic, multi-word chat_edit_title, no-arg pin_message, and the acronym edge) plus two non-topic backfill tests through the real _process_message (MessageActionChatEditTitle with a title, MessageActionPinMessage without).

No behavior change for users; this is a refactor + test/documentation pass. action_type/new_title remain unread by any consumer today (the viewer gates on service_type === 'service'), so values are unchanged on disk.

Testing

• Full suite: 1854 passed (1847 + 7 new) on Python 3.14
• ruff check . and ruff format --check . clean

Type of Change

• Refactor / internal quality (no functional change)
• Tests

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 07f6df69-bb6d-4300-bc52-54a16f2e857b

📥 Commits

Reviewing files that changed from the base of the PR and between a5bb6e4 and df6e505.

📒 Files selected for processing (3)

• src/message_utils.py
• src/telegram_backup.py
• tests/test_telegram_backup.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ai/service-action-review-fixes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.25%. Comparing base (a5bb6e4) to head (df6e505).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #192   +/-   ##
=======================================
  Coverage   92.25%   92.25%           
=======================================
  Files          25       25           
  Lines        7141     7141           
=======================================
  Hits         6588     6588           
  Misses        553      553           

Files with missing lines	Coverage Δ
src/message_utils.py	77.98% <100.00%> (+0.56%)	⬆️
src/telegram_backup.py	90.93% <100.00%> (-0.03%)	⬇️

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[github-actions]

🐳 Dev images published!

• drumsergio/telegram-archive:dev
• drumsergio/telegram-archive-viewer:dev

The dev/test instance will pick up these changes automatically (Portainer GitOps).

To test locally:

docker pull drumsergio/telegram-archive:dev
docker pull drumsergio/telegram-archive-viewer:dev