Restore Cloud Run metadata gzip compression

[anth-volk]

Fixes #3710

Summary

• Restore standard ASGI gzip compression so large legacy Flask responses, including /us/metadata, are transported compressed without changing the JSON payload shape.
• Add ASGI factory coverage showing large Flask fallback responses use HTTP gzip while callers still receive the normal decoded body.
• Force unit tests onto the legacy SQLite path before importing the SQL module, with a note that the SQL layer needs broader refactoring in a later migration stage.

Why

The Stage 3 Cloud Run staging integration run failed because /us/metadata exceeded Cloud Run's uncompressed response-size limit. The app logged 200 OK, but Cloud Run returned 500 with Response size was too large.

This keeps the existing endpoint contract intact: callers like policyengine-app-v2 can continue using normal fetch(...).json() with no custom decompression or framing.

Validation

• uv run pytest tests/unit/test_asgi_factory.py::test_large_flask_fallback_response_supports_http_gzip
• uv run ruff check tests/unit/conftest.py policyengine_api/asgi_factory.py tests/unit/test_asgi_factory.py
• uv run ruff format --check tests/unit/conftest.py policyengine_api/asgi_factory.py tests/unit/test_asgi_factory.py
• Manual Node fetch check against the patched ASGI wrapper confirmed fetch(...).json() receives decoded JSON while the transport response uses Content-Encoding: gzip.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.81%. Comparing base (0fc959c) to head (ab8156d).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3711      +/-   ##
==========================================
- Coverage   78.87%   78.81%   -0.06%     
==========================================
  Files          71       71              
  Lines        4318     4320       +2     
  Branches      804      804              
==========================================
- Hits         3406     3405       -1     
- Misses        698      702       +4     
+ Partials      214      213       -1     

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