load photos filmstrip once at the section layout

Created by: jedmund

Summary

Architecture audit A-12 — dedupe the filmstrip fetch across the photos section.

Previously, every visit to /photos/[id] re-fetched /api/photos?limit=100 for the filmstrip navigation, even when clicking through from another photo in the same section. Grid (/photos) did its own separate 20-item DB query alongside.

Now:

• New src/routes/photos/+layout.server.ts loads up to 100 photos once for the section and sets cache-control
• /photos/[id] reads photoItems from await parent() — no more per-navigation filmstrip fetch
• /photos grid slices its first page from parent instead of issuing a duplicate query (and dropping its own header-set to avoid conflict with the layout)

Impact

• Grid entry: 1 DB query (unchanged)
• Detail entry (deep link): 1 DB + 1 API fetch (was 2 API fetches)
• Detail → detail navigation: 0 filmstrip refetches (was 1 per navigation) — the main win

Test plan

• pnpm check — baseline 116 errors / 9 warnings, no new ones
• Visit /photos → grid renders first 20 photos with correct pagination/hasMore
• Click a photo → detail view loads; filmstrip shows all photos
• Navigate photo → photo via filmstrip → no /api/photos?limit=100 request in network tab
• Deep-link directly to /photos/[id] → filmstrip populates from layout load
• Infinite scroll / "load more" on the grid still works