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/base.html

@@ -49,6 +49,7 @@
       <li><a href="/"          class="nav-link {% if active == 'dashboard' %}active{% endif %}"><i class="bi bi-speedometer2 me-2"></i>Dashboard</a></li>
       <li><a href="/printers"  class="nav-link {% if active == 'printers'  %}active{% endif %}"><i class="bi bi-printer me-2"></i>Printers</a></li>
       <li><a href="/jobs"      class="nav-link {% if active == 'jobs'      %}active{% endif %}"><i class="bi bi-list-task me-2"></i>Jobs</a></li>
+      <li><a href="/brackets"  class="nav-link {% if active == 'brackets'  %}active{% endif %}"><i class="bi bi-collection me-2"></i>Brackets</a></li>
       <li><a href="/filaments" class="nav-link {% if active == 'filaments' %}active{% endif %}"><i class="bi bi-record-circle me-2"></i>Filaments</a></li>
       <li><a href="/todos"     class="nav-link {% if active == 'todos'     %}active{% endif %}"><i class="bi bi-check2-square me-2"></i>To-Do</a></li>
       <li><a href="/pricing"   class="nav-link {% if active == 'pricing'   %}active{% endif %}"><i class="bi bi-currency-euro me-2"></i>Pricing</a></li>