feat(05-02): create filterEntries utility function

- Add SearchFilters type and utilities (search.ts)
- Implement filterEntries with query, tags, type, dateRange filters
- Case-insensitive text search with 2-character minimum
- AND logic for tag filtering
- Date range inclusive of end date

src/lib/utils/filterEntries.ts

@@ -0,0 +1,55 @@
+import type { SearchFilters } from '$lib/types/search';
+import type { Entry, Tag } from '$lib/server/db/schema';
+
+interface EntryWithTags extends Entry {
+	tags: Tag[];
+}
+
+/**
+ * Filters entries based on search criteria.
+ *
+ * @param entries - Array of entries with their tags
+ * @param filters - SearchFilters object containing filter criteria
+ * @returns Filtered array of entries
+ */
+export function filterEntries(
+	entries: EntryWithTags[],
+	filters: SearchFilters
+): EntryWithTags[] {
+	let result = entries;
+
+	// Text search (title + content) - minimum 2 characters
+	if (filters.query.length >= 2) {
+		const query = filters.query.toLowerCase();
+		result = result.filter(
+			(e) =>
+				e.title?.toLowerCase().includes(query) || e.content.toLowerCase().includes(query)
+		);
+	}
+
+	// Tag filter - AND logic (must have ALL selected tags)
+	if (filters.tags.length > 0) {
+		result = result.filter((e) =>
+			filters.tags.every((tagName) =>
+				e.tags.some((t) => t.name.toLowerCase() === tagName.toLowerCase())
+			)
+		);
+	}
+
+	// Type filter
+	if (filters.type !== 'all') {
+		result = result.filter((e) => e.type === filters.type);
+	}
+
+	// Date range filter (using createdAt)
+	if (filters.dateRange.start) {
+		result = result.filter((e) => e.createdAt >= filters.dateRange.start!);
+	}
+	if (filters.dateRange.end) {
+		// Include the end date by comparing to end of day
+		const endOfDay = filters.dateRange.end + 'T23:59:59';
+		result = result.filter((e) => e.createdAt <= endOfDay);
+	}
+
+	return result;
+}