fix: Distinguish invalid upload tokens and warn about ephemeral encryption

[jan-auer]

Resumable upload tokens that cannot be decrypted or authenticated now return 400 with a generic error. Unknown sessions and object mismatches retain 404.

Startup emits a warning when persistent encryption keys are absent. The existing logging integration sends this warning to Sentry.

Ref FS-571

[linear-code]

FS-571

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.35%. Comparing base (61a8752) to head (48ad0cd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
objectstore-service/src/error.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #687   +/-   ##
=======================================
  Coverage   91.34%   91.35%           
=======================================
  Files         117      117           
  Lines       24173    24201   +28     
=======================================
+ Hits        22082    22109   +27     
- Misses       2091     2092    +1     

Components	Coverage Δ
Rust Backend	94.83% <95.00%> (+<0.01%)	⬆️
Rust Client	81.70% <ø> (ø)
Python Client	93.81% <ø> (ø)

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