fix(utils): make getStartEndDates validation guard actually validate

[arturasmckwcz]

Surfaced while reviewing the data-utils tests in #528.

getStartEndDates's guard was unsatisfiable:

if (startHour < endHour && startHour < 0 && startHour > 24 && endHour < 0 && endHour > 24)

startHour < 0 && startHour > 24 can never both hold, so invalid hours never threw. Replaced with the intended check — throw unless 0 <= startHour < endHour <= 24.

Scope: source-only. getStartEndDates has no production callers today (only the unit test in #528), so this is a correctness/hygiene fix with no runtime behavior change for live code, and the happy-path inputs in #528 (start < end, in range) still pass. The throw-path test belongs with the suite in #528, so this PR deliberately doesn't touch the test file (avoids a conflict with #528).

Not included: getStartEnd's map has questionable entries (e.g. "17:00-20:00" → 14..17), but it has 4 real callers and some entries look like intentional slot-snapping — correcting it would change live availability parsing, so it needs domain review, not a blind fix. Flagged on #528.

🤖 Generated with Claude Code

[arturasmckwcz]

Updated: in addition to fixing the unsatisfiable guard, getStartEndDates now throws BadRequestError instead of a raw Error. The global error handler (src/server/index.ts) only maps BaseError subclasses to their status code; a raw Error falls through to 500. Invalid hours are a bad-request case, so this now yields 400 — per the error-handling convention in CLAUDE.md.

[arturasmckwcz]

Changed direction per review: getStartEndDates is stray (no production callers — only its own definition + the barrel re-export, no tests on develop). Rather than fix a dead function's guard, this now removes it. Its sibling getStartEnd (4 callers) stays. Source-only; typecheck green.

[need4deed]

Approved: remove stray getStartEndDates (no production callers); getStartEnd retained. Source-only, typecheck green.