fix(files): store signatures without a public ACL; re-sign stored avatar URLs

[roncodes]

Part of making Fleetbase work with a fully private S3 bucket. Companion to fleetbase/core-api#287 (which has the full write-up).

• Signatures were not being stored on buckets with ObjectOwnership=BucketOwnerEnforced. ProofController::storeSignature() wrote with 'public' visibility (a public-read ACL). S3 rejects ACL'd PUTs on such buckets, and put() returned false without an error, so the proof and File records were created but no object existed. The write now goes out with no ACL; File::url serves it through a signed URL.
• avatar_url on Vehicle, Driver and Place: legacy rows that hold an absolute bucket URL are re-signed at read time via core-api File::signStoredUrl(). On core-api releases that predate the helper, the value passes through unchanged.

Tests: the signature-write fakes now assert that no visibility/ACL option is passed, and a pass-through test covers Utils::signStoredFileUrl.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9ba5d0d) to head (413aa47).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##                main      #353   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity     12379     12381    +2     
===========================================
  Files            600       600           
  Lines          46496     46498    +2     
===========================================
+ Hits           46496     46498    +2     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)

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