Use vector store for similarity search

[zhangzhenghao]

Summary

• write matrix factorization item embeddings into the vector store after collaborative filtering training
• store only matrix factorization user embeddings in blob storage, with backward-compatible worker loading for old blobs
• generate collaborative filtering recommendations via vector search instead of local item HNSW index
• use vector-store-backed query paths for embedding item-to-item recommendations

Tests

• go test ./logics ./master ./worker ./server -count=1

[codecov]

Codecov Report

❌ Patch coverage is 78.67647% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.11%. Comparing base (e57c32e) to head (c1e18c5).

Files with missing lines	Patch %	Lines
logics/writers.go	62.83%	26 Missing and 16 partials ⚠️
logics/item_to_item.go	87.19%	15 Missing and 11 partials ⚠️
logics/recommend.go	74.07%	7 Missing and 7 partials ⚠️
logics/user_to_user.go	86.31%	7 Missing and 6 partials ⚠️
server/rest.go	61.76%	8 Missing and 5 partials ⚠️
master/tasks.go	77.14%	4 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
- Coverage   72.17%   72.11%   -0.07%     
==========================================
  Files          94       95       +1     
  Lines       18608    18801     +193     
==========================================
+ Hits        13430    13558     +128     
- Misses       3764     3795      +31     
- Partials     1414     1448      +34     

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