feat(server): Add resumable upload API

[jan-auer]

Implements the resumable upload interface on the backend trait and the API endpoints. All backends decline by default, so session creation answers 409 Conflict and clients fall back to a regular upload.

Spec

Ref FS-470

[linear-code]

FS-470

[codecov]

Codecov Report

❌ Patch coverage is 95.47920% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (186c046) to head (77e2dab).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
objectstore-server/src/endpoints/resumable.rs	95.95%	10 Missing ⚠️
objectstore-service/src/backend/testing.rs	50.00%	8 Missing ⚠️
objectstore-service/src/error.rs	0.00%	3 Missing ⚠️
objectstore-types/src/resumable.rs	95.77%	3 Missing ⚠️
objectstore-server/src/endpoints/common.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #593      +/-   ##
==========================================
+ Coverage   87.91%   88.17%   +0.26%     
==========================================
  Files          95       97       +2     
  Lines       15850    16400     +550     
==========================================
+ Hits        13934    14461     +527     
- Misses       1916     1939      +23     

Components	Coverage Δ
Rust Backend	92.51% <95.47%> (+0.14%)	⬆️
Rust Client	81.60% <ø> (ø)
Python Client	91.51% <ø> (ø)

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