Phase 5: library & stats

- Full library home page: question list with per-user mastery stats,
  search/source filter, sort (A-Z, weakest first, most-seen), source
  breakdown in header, Take a test CTA.
- GET/POST /questions/{id}: view and edit question text, source, answers;
  radio-select correct answer; shows seen×/correct% stat.
- POST /questions/{id}/delete: hard delete (cascades to answers via FK).
- repo: ListQuestions supports SortWeakest/SortMostSeen via LEFT JOIN;
  added CountBySource, UpdateQuestion, UpdateAnswers, DeleteQuestion.
- render: added pct template func (correct*100/seen).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/question.html

@@ -0,0 +1,71 @@
+{{define "content"}}
+<div class="mb-5 flex items-center justify-between">
+  <a href="/" class="text-sm text-gray-500 hover:underline">← Library</a>
+  {{if .Stat}}
+  <p class="text-xs text-gray-500">
+    Seen {{.Stat.TimesSeen}}× · {{.Stat.TimesCorrect}} correct ({{pct .Stat.TimesCorrect .Stat.TimesSeen}}%)
+  </p>
+  {{else}}
+  <p class="text-xs text-gray-400">Not yet seen in a test</p>
+  {{end}}
+</div>
+
+<form method="post" action="/questions/{{.Question.ID}}" class="space-y-4">
+  <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
+
+  <div>
+    <label class="block text-sm font-medium text-gray-700 mb-1">Question</label>
+    <textarea name="q_text" rows="4" required
+              class="w-full border border-gray-300 rounded-md px-3 py-2 text-sm shadow-sm
+                     focus:outline-none focus:ring-2 focus:ring-blue-500 resize-y">{{.Question.Text}}</textarea>
+  </div>
+
+  <div>
+    <label class="block text-sm font-medium text-gray-700 mb-1">Source</label>
+    <input type="text" name="source" value="{{.Question.Source}}"
+           class="w-full border border-gray-300 rounded-md px-3 py-1.5 text-sm shadow-sm
+                  focus:outline-none focus:ring-2 focus:ring-blue-500">
+  </div>
+
+  <div>
+    <label class="block text-sm font-medium text-gray-700 mb-2">Answers</label>
+    <div class="space-y-2">
+      {{range $i, $a := .Answers}}
+      <div class="flex items-center gap-3">
+        <input type="radio" name="correct" value="{{$i}}"
+               {{if $a.IsCorrect}}checked{{end}}
+               class="text-blue-600 focus:ring-blue-500 flex-shrink-0">
+        <input type="text" name="a_text_{{$i}}" value="{{$a.Text}}" required
+               class="flex-1 border border-gray-300 rounded-md px-3 py-1.5 text-sm shadow-sm
+                      focus:outline-none focus:ring-2 focus:ring-blue-500">
+      </div>
+      {{end}}
+    </div>
+    <p class="text-xs text-gray-400 mt-1">Select the radio button next to the correct answer.</p>
+  </div>
+
+  <div class="flex gap-3 pt-2">
+    <button type="submit"
+            class="flex-1 bg-blue-600 hover:bg-blue-700 text-white py-2.5 px-4 rounded-md
+                   text-sm font-semibold shadow-sm">
+      Save changes
+    </button>
+    <a href="/"
+       class="px-4 py-2.5 border border-gray-300 rounded-md text-sm font-medium text-gray-700
+              hover:bg-gray-50 text-center">
+      Cancel
+    </a>
+  </div>
+</form>
+
+<div class="mt-6 pt-6 border-t border-gray-200">
+  <form method="post" action="/questions/{{.Question.ID}}/delete">
+    <input type="hidden" name="csrf_token" value="{{.CSRFToken}}">
+    <button type="submit"
+            onclick="return confirm('Delete this question? This cannot be undone.')"
+            class="text-sm text-red-600 hover:text-red-800 hover:underline">
+      Delete this question
+    </button>
+  </form>
+</div>
+{{end}}