fix(backend): run the test suite in forked processes so CI stops being OOM-killed

[Makisuo]

What broke

Main's CI went red on 3b300e6 with no failing test: the `test-packages` shard was killed with exit 137 while only `@maple/backend` was still running, ~4.5 minutes into a suite that normally finishes in ~4.7. The deploy gate then refused to ship. Re-running the job passed and the deploy went out, so this is a memory flake, not a regression from the commits above it (neither touches the backend package).

Why

The backend suite boots ~530 embedded PGlite instances across ~140 files. Under `pool: "threads"` every worker thread lives in the one vitest process, and on the Linux runner that process grew past 16 GB by the end of the run.

Measured locally on the full backend suite at one worker:

pool	peak RSS	floor between files
threads	2.6 GB	1.3 GB
forks	3.1 GB	0.7 GB

Neither pool leaks across files on macOS, so the runaway growth is specific to the Linux allocator holding on to memory freed by terminated threads. A forked worker hands everything back to the OS when the process exits, which threads cannot do. Threads were chosen for ~3 s of wall time on the full suite; that is not worth a red main.

Change

`packages/backend/vitest.config.ts`: `pool: "threads"` → `pool: "forks"`, `isolate` unchanged, comment updated with the evidence.

🤖 Generated with Claude Code

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Summary by CodeRabbit

• Tests
  • Test execution now runs in forked processes, improving test-runner compatibility while preserving existing test settings.

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ℹ️ Recent review info

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• packages/backend/vitest.config.ts

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📝 Walkthrough

Walkthrough

Vitest now runs backend tests in forked processes instead of worker threads. Other test settings remain unchanged.

Changes

Vitest pool configuration

Layer / File(s)	Summary
Select forked test processes packages/backend/vitest.config.ts	The Vitest test pool changes from worker threads to forked processes. The configuration comment describes process isolation behavior.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: using forked processes for backend tests to prevent CI out-of-memory termination.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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