perf(backend): index roms (platform_id, fs_size_bytes)

[gantoine]

Description

GET /api/platforms (11.5s) and GET /api/stats (~5s) slow down badly as the library grows because both sum roms.fs_size_bytes (per-platform via the Platform.fs_size_bytes column property, and whole-library for stats). fs_size_bytes was not part of any index, so the database had to read actual rows from the very wide, JSON-heavy roms table (964 MB / 92,800 roms in the reporter's case). rom_count on the same model is instant because COUNT is satisfied by the existing platform_id index.

This PR adds a composite index on roms (platform_id, fs_size_bytes), turning both sums into index-only scans that never touch the wide rows. The optimizer picks the covering index for both query shapes on its own, so no query changes are needed.

Changes:

• backend/models/rom.py: add Index("idx_roms_platform_fs_size", "platform_id", "fs_size_bytes") to Rom.__table_args__.
• backend/alembic/versions/0097_roms_platform_fs_size_index.py: migration creating the index, using batch_alter_table + if_not_exists/if_exists for MariaDB/MySQL/PostgreSQL compatibility (mirrors the existing 0093 index migration).

Reported measurements (from the issue and a member's replica of a 92,800-rom production DB):

Query	Before	After
Per-platform SUM(fs_size_bytes) + COUNT (/api/platforms shape)	5.15s	0.07s
Whole-library SUM(fs_size_bytes) (/api/stats shape)	4.06s	0.03s

Index build took ~4.7s on the 964 MB table, so the migration is cheap even for large libraries.

Fixes #3769

AI-assistance disclosure: This change was written with AI assistance (PostHog Code / Claude). The diagnosis, index design, and migration were AI-generated; the timings in the issue and above were verified by a maintainer with read-only queries against a production-scale database.

Checklist

• 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 (index-only change; see timing table above)

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Created with PostHog Code

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[greptile-apps]

Greptile Summary

This PR adds a covering index for ROM size aggregation. The main changes are:

• Adds an Alembic migration for roms(platform_id, fs_size_bytes).
• Declares the same composite index in the ROM model.
• Provides reversible, idempotent index operations.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The revision chain and migration API match existing repository patterns.
• The model and migration define the same index.

Important Files Changed

Filename	Overview
backend/alembic/versions/0097_roms_platform_fs_size_index.py	Adds a reversible composite-index migration using the same batch-operation pattern as existing migrations.
backend/models/rom.py	Adds the matching composite index to SQLAlchemy metadata.

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