feat: implement deletion, JSON encoding, and query optimizations for SQLite storage

[dundee]

Summary

Implements the previously stubbed-out RemoveFile, RemoveFileByName, and
EncodeJSON methods for the SQLite storage backend, along with performance
improvements to the deletion path and query indexing.

Changes

Deletion (RemoveFile, RemoveFileByName)

• Recursive CTE deletes an item and all its descendants in a single SQL statement
• Ancestor stats (size, usage, item_count) are updated per-ancestor within the
  same transaction, using a single in-memory walk — no redundant recursive CTE
  for ancestor updates
• In-memory stats are only applied after the transaction commits, preventing
  DB/memory inconsistency on rollback
• GetParentLocked() avoids deadlock by traversing cached parents without
  acquiring storage.m.RLock while storage.m.Lock is held
• GetChildByName enables RemoveFileByName to look up children by name

JSON encoding (EncodeJSON)

• encodeDirJSON recursively encodes directories and their children
• encodeFileJSON encodes files including hard-link and non-regular flags
• Matches the output format of the in-memory Dir/File EncodeJSON methods

Query optimizations

• Added composite index (parent_id, name) for GetChildByName lookups
• Removed unused DeleteItem public method (its logic is covered by the
  internal deleteItemTree)

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[codecov]

Codecov Report

❌ Patch coverage is 66.82692% with 69 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.90%. Comparing base (8f8a6bc) to head (a3e11c5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/analyze/sqlite.go	66.82%	46 Missing and 23 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
+ Coverage   84.36%   85.90%   +1.53%     
==========================================
  Files          52       52              
  Lines        5566     5774     +208     
==========================================
+ Hits         4696     4960     +264     
+ Misses        722      640      -82     
- Partials      148      174      +26     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.