Replace serde_json with postcard for cache serialization

[jserv]

Switch cache entry encoding from JSON to postcard binary format. Postcard uses varint encoding, producing more compact entries by eliminating JSON field names and syntax overhead. This increases effective cache capacity within the same max_size_mb limit.

Legacy JSON entries are automatically evicted on first access (deserialization failure triggers db.remove), avoiding dead weight in the sled database.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 68.40%. Comparing base (2616bee) to head (7665dc8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cache.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   68.48%   68.40%   -0.09%     
==========================================
  Files          11       11              
  Lines         768      769       +1     
==========================================
  Hits          526      526              
- Misses        242      243       +1     

Flag	Coverage Δ
unittests	68.40% <66.66%> (-0.09%)	⬇️

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Files with missing lines	Coverage Δ
src/cache.rs	59.59% <66.66%> (-0.61%)	⬇️

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