Fix: admin firework/multishot saves time out from unscoped timeline-safety triggers

[liammaloney2403]

Saving a firework or multishot shot in Admin could time out because the database's timeline-safety triggers re-validated every show and every published preset on every write, instead of just the ones the write could affect.

• Root cause: assert_show_timeline_non_overlapping() (3.6s) and assert_all_published_show_presets() (2.6s) ran unscoped on every catalogue_items / fireworks / multishot_fireworks write; fan-out repeated this once per multishot reusing an edited firework (reproduced a >60s hang on a firework used in 17 multishots)
• Fix: migration platform/supabase/migrations/20260810120000_scope_show_timeline_safety_checks.sql scopes both checks to the shows/catalogue items actually affected, and adds WHEN guards so duration-propagation triggers skip when nothing relevant changed
• Verified: full unscoped audit still passes (0 overlaps across 67 shows / 163 published presets), existing 27-test overlap-safety suite passes unchanged, and a genuine unsafe duration change is still correctly rejected
• Result: firework save >60s timeout → 274ms; multishot shot save 9.0s → 246ms
• Testing log: platform/docs/firework-timeline-safety-performance.md

[linear]

FIR-174