fix: release database lock during file I/O in cleanup_deleted

[laynepenney]

Summary

Fixes lock contention issue identified in PR #285 code review.

Problem

In cleanup_deleted(), the database lock was held during file system operations:

let db_lock = db.lock().await;  // Lock acquired
for file_path in indexed_files {
    if !full_path.exists() {  // Blocking I/O under lock
        db_lock.delete_file(file.id)?;
    }
}

This blocks other operations from accessing the database during file system checks.

Solution

Refactored to release the lock during file I/O:

1. Get indexed files (lock held briefly)
2. Collect paths to delete (no lock - file system checks)
3. Delete files (lock held briefly per operation)

Changes

• codi-rs/src/symbol_index/indexer.rs - Refactored cleanup_deleted()

Verification

• Zero build warnings
• All 516 tests pass
• Clean build

Related

• Fixes issue identified in PR feat(phase-2): implement file cleanup and fix warnings #285 code review
• Part of Production Readiness Phase 2

[laynepenney]

Closing this PR. After further review, the lock contention 'fix' is unnecessary - the original code from PR #285 is acceptable for a manual maintenance operation. The 'fix' introduced redundant queries and unnecessary complexity for a theoretical problem that doesn't exist in practice.