Add job brackets for grouping multi-part or repeated jobs

- JobBracket model: name, customer, notes
- Migration 0006: job_brackets table + bracket_id FK on print_jobs
- /brackets: list with per-bracket progress bar
- /brackets/{id}: detail with jobs table + Add Job(s) modal (quantity field
  creates N identical jobs in one shot, each with its own UUID and log entry)
- /jobs Add Job modal: optional bracket selector
- Sidebar: Brackets nav item added

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

app/templates/jobs.html

@@ -152,6 +152,16 @@
             </select>
           </div>
 
+          <div>
+            <label class="form-label text-secondary small">Bracket <span class="text-secondary fw-normal">(optional)</span></label>
+            <select name="bracket_id" class="form-select bg-dark border-secondary text-white">
+              <option value="">— none —</option>
+              {% for b in brackets %}
+              <option value="{{ b.id }}">{{ b.name }}{% if b.customer %} · {{ b.customer }}{% endif %}</option>
+              {% endfor %}
+            </select>
+          </div>
+
           <div>
             <label class="form-label text-secondary small">Notes</label>
             <textarea name="notes" class="form-control bg-dark border-secondary text-white"