refactor(db): move Pebble context under db common

[zhangzhenghao]

Summary

• move the shared Pebble index-store wrapper from internal/indexstore/indexstore.go to internal/db/common/pebble_context.go
• keep the Go package name as common
• update FTS and inverted-index consumers without changing persistence behavior or disk markers

Verification

• go test ./...
• go test -race ./...
• go vet ./...
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.39%. Comparing base (a6dbfe2) to head (138e6a4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #24   +/-   ##
=======================================
  Coverage   61.39%   61.39%           
=======================================
  Files         155      155           
  Lines       40415    40415           
=======================================
  Hits        24812    24812           
  Misses      12366    12366           
  Partials     3237     3237           

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