fix(playmatch): handle empty heartbeat query

[DevYukine]

Description
The 5.0 metadata page checks the Playmatch health endpoint as soon as Playmatch is enabled. That check currently fails before sending a request because RomM expands an empty query into yarl.URL.update_query(), which yarl rejects.

This passes the query dictionary directly instead. Empty heartbeat queries now keep the original health URL, while normal identify queries still add their parameters.

Also adds a regression test for the Playmatch heartbeat request.

Disclosure
I am not a Python developer and am not very familiar with the RomM codebase. I used Codex using GPT 5.6 Sol to help with the Playmatch heartbeat bugfix and to validate the regression test. I reviewed the result before submitting.

Checklist
^{Please check all that apply.}

• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

Backend change only, no screenshots needed

[greptile-apps]

Greptile Summary

This PR fixes Playmatch heartbeat requests with an empty query. The main changes are:

• Pass the filtered query mapping directly to yarl.URL.update_query().
• Preserve the query-free health endpoint for heartbeat requests.
• Add a test for the heartbeat HTTP request.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
backend/handler/metadata/playmatch_handler.py	Allows an empty query mapping while preserving populated identify-query behavior.
backend/tests/handler/metadata/test_playmatch_handler.py	Tests that heartbeat calls the health endpoint without query parameters.

_{Reviews (1): Last reviewed commit: "fix(playmatch): handle empty heartbeat q..." | Re-trigger Greptile}