perf(roms): make full rom id index opt-out on GET /api/roms

[gantoine]

Description
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Fixes #3770

The home page rails ("recently added" and "continue playing") call GET /api/roms?limit=15&order_by=id&order_dir=desc. That endpoint always computes rom_id_index, the complete ordered list of every matching rom id, which exists to back the gallery's virtual scroll. On a large library (the reporter has ~92,800 roms) this means:

• a full-table scan (SELECT id FROM roms ORDER BY id DESC) that took ~5.3s uncached, and
• ~600 KB of JSON (one integer per rom) shipped just to render 15 cards.

There were already opt-outs for the other sidecars (with_char_index, with_filter_values) but none for rom_id_index.

Changes

• Backend — add a with_rom_id_index query param to GET /api/roms, defaulting to true for backwards compatibility (mirroring the existing sidecar flags). When false, the full index is skipped and the requested page is served directly with offset/limit off the sort index, with a lightweight COUNT(*) for total. Added a get_rom_count() handler helper for the count. Default behavior (virtual-scroll gallery) is unchanged.
• Frontend — the home rail fetches (getRecentRoms / getRecentPlayedRoms, plus their cache-clear mirrors) now pass with_rom_id_index: false.

A 15-card rail now costs a small page query plus a count, and a few KB of payload.

The response schema is unchanged (rom_id_index is still present, just empty when opted out), so no OpenAPI/type regeneration is needed.

AI assistance disclosure

This change was written with AI assistance (PostHog Code / Claude). The implementation, tests, and verification were AI-generated and human-reviewed.

Checklist
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• 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)

N/A (backend/data-fetching change, no visual difference).

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Created with PostHog Code

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[greptile-apps]

Greptile Summary

This PR lets small ROM requests skip the full ROM ID index. The main changes are:

• Adds the backward-compatible with_rom_id_index query parameter.
• Uses direct pagination and a count when the index is disabled.
• Disables the index for recent and continue-playing home rails.
• Keeps matching cache-clear keys and adds endpoint coverage.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Existing callers retain the original behavior by default.
• Home-rail consumers only use page items and tolerate the empty index.

Important Files Changed

Filename	Overview
backend/endpoints/roms/init.py	Adds the opt-out parameter and selects between ID-index pagination and direct database pagination.
backend/handler/database/roms_handler.py	Adds a helper that counts the rows represented by the filtered ROM query.
backend/tests/endpoints/roms/test_rom.py	Tests the empty index, total, and page contents for an opted-out request.
frontend/src/services/api/rom.ts	Disables the full ID index for recent and recently played ROM requests.
frontend/src/services/cache/api.ts	Applies the opt-out consistently to cached requests and cache invalidation patterns.

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