Add run tagging (DB column + migration + route + UI)

- Added tags VARCHAR(300) column to Run model
- Added startup ALTER TABLE migration in app.py
- New POST /run/<id>/set_tags route saves comma-separated tags
- Tag badges + collapsible edit form in run_details.html header area

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

templates/run_details.html

@@ -108,6 +108,28 @@
     </div>
 </div>
 
+<!-- Tags -->
+<div class="mb-3 d-flex align-items-center gap-2 flex-wrap">
+    {% if run.tags %}
+        {% for tag in run.tags.split(',') %}
+            <span class="badge bg-secondary fs-6">{{ tag }}</span>
+        {% endfor %}
+    {% else %}
+        <span class="text-muted small fst-italic">No tags</span>
+    {% endif %}
+    <button class="btn btn-sm btn-outline-secondary" data-bs-toggle="collapse" data-bs-target="#tagsForm">
+        <i class="fas fa-tag me-1"></i>Edit Tags
+    </button>
+    <div class="collapse w-100" id="tagsForm">
+        <form action="{{ url_for('run.set_tags', id=run.id) }}" method="POST" class="d-flex gap-2 mt-1">
+            <input type="text" name="tags" class="form-control form-control-sm"
+                   value="{{ run.tags or '' }}"
+                   placeholder="comma-separated tags, e.g. dark-ending, v2, favourite">
+            <button type="submit" class="btn btn-sm btn-primary">Save</button>
+        </form>
+    </div>
+</div>
+
 <!-- Status Bar -->
 <div class="card shadow-sm mb-4">
     <div class="card-body">