fix: make sponsor coach spots nil-safe

[mroderick]

Problem

Linked to #2500. The admin sponsor show page returned an HTTP 500 for some sponsor records.

Sponsor#coach_spots did number_of_coaches || (seats / 2.0).round, which raises NoMethodError: undefined method '/' for nil when both number_of_coaches and seats are nil. The show view calls this unconditionally, so any incomplete record crashes.

The #1794 fix (adding presence validations + HTML5 required) prevents new bad rows, but pre-existing records can still hold nil values, so coach_spots remains crash-prone.

Change

Make coach_spots nil-safe: return number_of_coaches when present, else half the seats rounded, else nil instead of raising.

Tests

Added three #coach_spots cases to spec/models/sponsor_spec.rb. The nil-both case fails on the old implementation and passes with this fix.

Database analysis

Checked the codebar_dump Postgres database for records that would trigger this crash. Querying for sponsors rows where seats IS NULL OR number_of_coaches IS NULL returned 0 rows across all 751 sponsors. The broken records referenced in #2500 (IDs 1539 and 1540) are absent from the dump — only the valid 1541 (BDO UK, seats 0 / coaches 0) survives — so the bad rows appear to have already been removed in production. No data cleanup is required. The 0 values on remaining rows are valid (validation allows >= 0), so this fix is defensive hardening against recurrence through any non-controller data path.

Note: the "duplicate sponsor on update" aspect of #2500 was not reproducible — no code path in the update/create actions creates a new Sponsor row.