feat: add SQLite vector retrieval example and embeddings docs updates

[leehack]

Summary

• Add an end-to-end basic_app SQLite vector retrieval CLI that supports exact search, quantized ANN search, score interpretation, and quantized-vs-exact quality checks.
• Refactor the example into reusable CLI-options and vector-search services under lib/services/, and add focused unit tests for option parsing, score filtering, and recall metrics.
• Update top-level and website docs/changelog to align embedding support messaging (including web bridge requirements) and document the new SQLite vector workflow.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.81%. Comparing base (0952a27) to head (dbeaf6f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #76   +/-   ##
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  Coverage   76.81%   76.81%           
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  Files          66       66           
  Lines        8579     8579           
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  Hits         6590     6590           
  Misses       1989     1989           

Flag	Coverage Δ
unittests	76.81% <ø> (ø)

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