Fix: Replace firebase PENDING gate with version-based idempotency

[susilnem]

The PENDING status guard caused a race: a stats push (only_stats=True) and a status-change push racing for the same project would leave the second push rejected as FAILED, keeping a FINISHED project visible in Firebase.

Problem

When a stats push (only_stats=True) and a status-change push raced for the
same project, the second push hit the PENDING guard and was rejected as FAILED,
leaving a FINISHED project still visible in Firebase.

Changes

Replaced the PENDING status gate with two version counters on Project:

• firebase_push_requested_version — bumped atomically by every push trigger
• firebase_pushed_version — recorded on success using the snapshot taken at
  task start (not a re-read), so a concurrent bump can't be marked as satisfied
  with stale data

The push task:

1. Skips if pushed_version >= requested_version (already up-to-date, no false FAILED)
2. Re-enqueues on success if requested_version grew mid-push — heals the silent
   lock-drop path where a concurrent task is dropped while the lock is held

This PR doesn't introduce any:

• temporary files, auto-generated files or secret keys
• n+1 queries
• flake8 issues
• print
• typos
• unwanted comments

This PR contains valid:

• tests
• permission checks (tests here too)
• translations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.68%. Comparing base (189901a) to head (e987cfa).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #267      +/-   ##
===========================================
+ Coverage    87.55%   87.68%   +0.13%     
===========================================
  Files          211      212       +1     
  Lines        12819    12907      +88     
  Branches      1089     1090       +1     
===========================================
+ Hits         11224    11318      +94     
+ Misses        1196     1192       -4     
+ Partials       399      397       -2     

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