Draft: Backend implementation for server-side image builds

[AdrianoKF]

This PR adds a prototype for the backend API for server-side image builds.

Usage:

# Put together build context (in client/)
client/$ tar --exclude-from=../.gitignore -czf context.tgz .

# Submit the build request
client/$ curl -X 'POST' \
  'http://localhost:8000/builds/build' \
  -H 'accept: application/json' \
  -H 'Content-Type: multipart/form-data' \
  -F 'image_spec=@examples/example-hello.yaml;type=application/x-yaml' \
  -F 'build_context=@context.tgz;type=application/x-gzip' \
  -F 'name=test-image' \
  -F 'tag=latest' \
  -F 'platform=linux/amd64'

See issue #43

[codecov]

Codecov Report

Attention: Patch coverage is 45.41063% with 226 lines in your changes missing coverage. Please review.

Project coverage is 55.28%. Comparing base (abdc993) to head (4ab26c2).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ckend/src/jobq_server/utils/assembler/renderers.py	40.42%	112 Missing ⚠️
backend/src/jobq_server/utils/assembler/config.py	54.36%	47 Missing ⚠️
backend/src/jobq_server/routers/builds.py	51.28%	38 Missing ⚠️
backend/src/jobq_server/utils/processes.py	29.26%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   56.60%   55.28%   -1.33%     
==========================================
  Files          62       67       +5     
  Lines        3132     3545     +413     
==========================================
+ Hits         1773     1960     +187     
- Misses       1359     1585     +226     

Flag	Coverage Δ
backend	71.26% <45.41%> (-17.01%)	⬇️

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[maxmynter]

We could use StrEnum instead of Str, Enum server side, right?

[AdrianoKF]

I don't see this code ending up in the real implementation, but in principle yes :)