feat: enhance search with date-prioritized year filtering and true exact matching

Improve media item search functionality with two key enhancements:

1. Date-prioritized year filtering:
   - Prioritize date_published over copyright_year for year range queries
   - Fall back to copyright_year when date_published is NULL
   - Extract year from date_published timestamp for comparison

2. True exact search matching:
   - Replace ILIKE pattern matching with exact equality for quoted queries
   - Use search_query directly instead of wildcard pattern for exact matches
   - Remove SearchPattern parameter and related wildcard logic
   - Add COALESCE handling for author/series NULL values in exact matches

These changes make year filtering more accurate with published dates
and provide genuine exact matching when users wrap queries in quotes.

Refs internal/database/queries/queries.sql:475, internal/services/search.go:62

internal/router/frontend.go

@@ -208,10 +208,10 @@ func registerFrontendRoutes(cfg *Config) {
 					HasCover:       pgtype.Bool{Bool: false, Valid: true}, // Changed
 					SearchQuery:    pgtype.Text{String: "", Valid: true},  // NEW: Required
 					IsExactSearch:  pgtype.Bool{Bool: false, Valid: true}, // NEW: Required
-					SearchPattern:  pgtype.Text{Valid: false},             // NEW: Empty for no exact search
-					Sort:           pgtype.Text{String: "created_at DESC", Valid: true},
-					Limit:          pgtype.Int4{Int32: int32(limit), Valid: true},
-					Offset:         pgtype.Int4{Int32: int32(offset), Valid: true},
+					// SearchPattern:  pgtype.Text{Valid: false},             // NEW: Empty for no exact search
+					Sort:   pgtype.Text{String: "created_at DESC", Valid: true},
+					Limit:  pgtype.Int4{Int32: int32(limit), Valid: true},
+					Offset: pgtype.Int4{Int32: int32(offset), Valid: true},
 				})
 
 				if err != nil {