fix: cap transaction page size

[silentgeckoaudit3801]

Summary

• reduce the transaction list MAX_LIMIT from 500 to 50
• add a static regression test that locks the intended cap and rejects the old large value

Fixes #1335.

Validation

• Static source inspection only; no local dependency install or test execution in this environment.

[ogazboiz]

the fix itself is right, MAX_LIMIT 500 to 50 matches the issue and the Math.min path is verified. one tiny thing now blocks it: CI lint fails with a missing trailing newline in backend/src/controllers/tests/transactionControllerLimits.test.ts (prettier "Insert newline" at 12:4). run:

cd backend && npx prettier --write src/controllers/__tests__/transactionControllerLimits.test.ts && git commit -am "chore: prettier" && git push

and this merges.

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