Add response schema stability test for /api/subscriptions

[arisu6804]

Closes #668

Summary

Added tests/schema/subscriptions.test.ts, a snapshot-based response-shape stability suite for /api/subscriptions, following this repo's existing tests/schema/ convention (e.g. rate-limit.test.ts, alerts.test.ts). Pins the exact JSON returned for:

• GET / — list shape (secret stripped) and empty-list shape
• POST / — creation shape (secret included once)
• POST / — validation-error shape
• GET /:id — single-resource shape and not-found error shape

Two response fields are non-deterministic per request outside this test's control — the freshly generated secret (a uuidv4() minted per POST /) and correlationId on error responses (a randomUUID() fallback outside requestContextStorage) — both are pinned with expect.any(String) property matchers rather than snapshotted by value, so the suite stays deterministic across runs while still catching shape drift.

This complements the existing behavioral coverage in tests/subscriptions.test.ts (explicit field-by-field assertions for CRUD + validators) rather than duplicating it — the new file's only job is catching accidental shape drift (a renamed/added/removed field) via snapshot diff.

Verification

• npx jest tests/schema/subscriptions.test.ts — 6/6 pass, run twice back-to-back to confirm the snapshots are stable (second run reports "6 passed" against the committed snapshot file, not "6 written").
• npx jest tests/subscriptions.test.ts (pre-existing file, unmodified) — 67/68 pass; one pre-existing failure (DELETE /:id expects 204, receives 404) reproduces identically on main (git diff main --stat shows zero modifications to any tracked file in this PR), unrelated to this change.
• npx tsc --noEmit (project config): only the pre-existing src/routes/users.ts syntax errors present identically on main.
• npx eslint tests/schema/subscriptions.test.ts — clean, no errors.