feat(dashboard): implement Phase 4.5 Collections Preview Endpoint

Add preview endpoint for custom section builder and rule evaluation:

Handler Implementation (internal/handlers/collections.go):
- PreviewCollection method: Evaluates filter rules and returns matching items without saving
  * Accepts library_id, rules array, manual_book_ids array, and limit
  * Evaluates rules against all library items using collectionService.EvaluateRules
  * Adds manually selected books to results
  * Deduplicates manual books (avoids adding same book twice)
  * Applies limit (default: 20, max: 100)
  * Returns array of BookInfo with matching items
- Helper function: mediaItemsToListMediaItemsRow
  * Converts database.MediaItems to database.ListMediaItemsRow
  * Required for EvaluateRules which expects ListMediaItemsRow type

Route Registration (internal/router/collections.go):
- POST /api/collections/preview
- Protected by JWT middleware
- Part of collections API group

Why This Endpoint is Necessary:
- Allows users to see what books match their filter rules BEFORE saving
- Avoids creating incorrect collections
- Enables testing different rule combinations quickly
- Reuses existing service logic (collectionService.EvaluateRules)
- Client-side preview would require downloading entire library (10,000+ books)
- Would duplicate 500+ lines of rule evaluation logic in TypeScript
- Would create maintenance nightmare keeping Go and TypeScript in sync

Bruno Test (bruno/collections/preview-collection.bru):
- Tests POST /api/collections/preview endpoint
- Validates status 200 response
- Validates items array in response
- Example request with genre filter rule

This endpoint is required for both the web UI Custom Section Builder and future mobile apps.

internal/router/collections.go

@@ -20,6 +20,7 @@ func registerCollectionsRoutes(cfg *Config) {
 	collections.POST("/:id/books/bulk-remove", cfg.CollectionHandler.BulkRemoveBooks)
 	collections.POST("/bulk-add-books", cfg.CollectionHandler.HandleBulkAddBooks)
 	collections.POST("/test-rules", cfg.CollectionHandler.TestRules)
+	collections.POST("/preview", cfg.CollectionHandler.PreviewCollection)
 
 	// Device shelf mapping routes
 	deviceCollections := protected.Group("/devices/:id/collections")