BE030: Meal-Plan Request Bounds, Concurrency, Timeout

[shshwtsuthar]

Ticket

• BE030

Description

• Validates the meal-plan request schema and strict product/body limits.
• Caps classification work at four concurrent requests, with a 2-second classifier timeout and 12-second request deadline.
• Aborts in-flight classifier fetches on classifier timeout, request timeout, or client cancellation.
• Excludes individual classification failures with a stable partial-plan warning; returns sanitized failures when no classification succeeds or the request times out.
• Adds load/fault regression coverage and a timing/concurrency evidence note.

Validation

• npm test -- --runInBand (15 suites, 206 tests passed)
• 100-candidate regression test observed exactly four active classifier fetches.

Coordination

• BE018 also touches this route. This PR is rebased on main and limits itself to request bounds, timeout, concurrency, and error handling.

Checks

• All requirements of the ticket have been implemented, or I have commented on any exclusions
• Unit tests have been added or updated for any backend changes (if applicable)
• I have reviewed the Files Changed tab and verified it only contains relevant changes
• This PR has been reviewed and approved