feat(collections): implement rule testing/preview functionality (Limitation #1)

Add ability to test collection rules before saving:
- New API endpoint: POST /api/collections/test-rules
- Evaluates rules against all media items
- Returns matching books with reasons
- Supports all operators: equals, contains, greater_than, etc.
- Works with all fields: genre, author, series, etc.

Backend Implementation:
- TestRules() handler in collections.go
- evaluateRule() matches book properties against rule criteria
- compareValues() handles string/numeric comparisons
- Case-insensitive matching for contains operator

Frontend Integration:
- Updated testRule() function in collection_rules.templ
- Displays matching books with covers and authors
- Shows match reason (which rule criteria matched)
- Limits preview to 20 results with count indicator

Tests Added:
- TestCompareValues_Equals: Exact match validation
- TestCompareValues_Contains: Substring matching
- TestCompareValues_GreaterThan: Numeric comparison
- TestCompareValues_NotEquals: Negation
- TestEvaluateRule_Genre: Genre field matching
- TestEvaluateRule_Author: Author field matching
- TestEvaluateRule_CopyrightYear: Year field matching

API Request Format:
{
  "rules": [{
    "field": "genre",
    "operator": "equals",
    "value": "Science Fiction"
  }]
}

API Response Format:
{
  "matches": [{
    "media_item_id": "uuid",
    "title": "Book Title",
    "author": "Author Name",
    "cover_image_path": "/path/to/cover.jpg",
    "match_reason": "Matched rule: genre equals Science Fiction"
  }],
  "total": 42
}

Resolves Limitation #1: Rule Testing Preview

templates/collection_rules.templ

@@ -301,13 +301,19 @@ templ CollectionRules(user User, collection CollectionDetailData) {
                 testResultsDiv.classList.remove('hidden');
                 resultsList.innerHTML = '<p class="text-sm" style="color: var(--text-secondary)">Testing rule...</p>';
 
-                fetch(`/api/collections/${collectionId}/test-rule`, {
+                const rules = [{
+                    field: field,
+                    operator: operator,
+                    value: value
+                }];
+
+                fetch('/api/collections/test-rules', {
                     method: 'POST',
                     headers: {
                         'Content-Type': 'application/json',
                         'Authorization': 'Bearer ' + localStorage.getItem('token')
                     },
-                    body: JSON.stringify({ field, operator, value })
+                    body: JSON.stringify({ rules: rules })
                 })
                 .then(response => response.json())
                 .then(result => {
@@ -316,9 +322,15 @@ templ CollectionRules(user User, collection CollectionDetailData) {
                         html += '<div style="display: flex; flex-direction: column; gap: 0.5rem;">';
 
                         result.matches.slice(0, 20).forEach(book => {
-                            html += '<div style="display: flex; align-items: center; gap: 0.75rem; padding: 0.5rem; border-radius: 0.25rem; background-color: var(--bg-primary);">';
+                            html += '<div style="display: flex; align-items: center; gap: 0.75rem; padding: 0.5rem; border-radius: 0.25rem; background-color: var(--bg-secondary);">';
                             html += '<img src="' + (book.cover_image_path || '/static/placeholder-book.svg') + '" alt="Cover" style="width: 2rem; height: 3rem; object-fit: cover; border-radius: 0.25rem;">';
-                            html += '<span style="font-size: 0.875rem; font-weight: 500; color: var(--text-primary);">' + book.title + '</span>';
+                            html += '<div style="flex: 1;">';
+                            html += '<div style="font-size: 0.875rem; font-weight: 500; color: var(--text-primary);">' + book.title + '</div>';
+                            if (book.author) {
+                                html += '<div style="font-size: 0.75rem; color: var(--text-secondary);">' + book.author + '</div>';
+                            }
+                            html += '<div style="font-size: 0.75rem; color: var(--accent);">' + book.match_reason + '</div>';
+                            html += '</div>';
                             html += '</div>';
                         });