fix: add error checking for unchecked return values

[korjavin]

Summary

Fix errcheck issues across the codebase by properly handling unchecked return values.

Changes

Error handling fixes:

• conversation.go: Added error checking for tx.Rollback(), stmt.Close(), rows.Close() using deferred anonymous functions
• elevenlabs.go: Added error checking for resp.Body.Close()
• class_scheduler.go: Added error logging for cleanupOldClasses() return value
• main.go: Added error logging for db.Close()
• services.go: Added error checking for transaction and DB resource handling
• stat_handler.go: Added blank identifier assignment for bot.Send() errors
• cmd/tool-api/main.go: Added error checking for HTTP response handling
• importer/importer.go: Added error checking for file.Close() and qdrantClient.Close()

Pattern used:

For deferred cleanup calls that can't meaningfully handle errors:

defer func() { _ = stmt.Close() }()

For inline calls where errors are intentionally ignored:

_, _ = bot.Send(msg)

Testing

All tests pass: go test ./... ✓

Checklist

• Code compiles without errors
• All tests pass
• No new linter warnings introduced
• Changes follow existing code style