Persistent thumbnail color cache with ListRow pattern

[chrisballinger]

Summary

• Single-query list fetching: Replaces dual observations (items + favorites) with a single GRDB read transaction returning ListRow<T> that bundles the object + full ObjectMetadata + cached ThumbnailColors
• Persistent color cache: New thumbnail_colors GRDB table stores extracted thumbnail colors across app restarts — no more re-extracting from images on every launch
• Selective list row theming: Only Art and Mutant Vehicle rows use thumbnail-derived colors; Camp and Event rows use the default theme. Detail views use colors for all types.
• Pre-loaded detail views: DetailPageItem threads metadata + colors from list → detail, so detail views render instantly with no async flicker

New files

File	Description
ThumbnailColors.swift	Codable GRDB model (16 RGBA doubles)
ListRow.swift	Generic ListRow<T> — object + metadata + colors
ColorPrefetcher.swift	Background batch color extraction at launch
ThumbnailColors+UIKit.swift	Conversion helpers (↔ UIColor, ImageColors, BRCImageColors)
DetailPageItem	Pairs DetailSubject with pre-loaded metadata + colors

Architecture changes

• PlayaDB observation methods return [ListRow<T>] instead of [T]
• ObjectListViewModel stores [ListRow<Object>], eliminates separate favorites observation
• ObjectRowView accepts thumbnailColors param, gates on DisplayableObject.supportsColorTheming
• Thumbnail downloaders return awaitable Task<Set<String>, Never> with corrupt file validation
• DependencyContainer sequences: downloads → ColorPrefetcher.prefetchMissingColors()

Test plan

• Scroll Art list — rows show thumbnail-derived colors
• Scroll MV list — rows show thumbnail-derived colors
• Scroll Camp/Event lists — rows use default/plain theme (no image colors)
• Tap Art from list → detail view shows colors immediately (no flicker)
• Tap Camp from map → detail view still loads colors from DB cache
• Toggle favorite in list → instant optimistic update, persists correctly
• Kill + restart app → Art/MV list colors appear immediately from DB
• Swipe between detail pages → each shows correct colors + metadata

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