Add start, stop, and finish job actions

- POST /jobs/{id}/start: modal picks from idle/offline printers,
  sets job → printing, printer → printing, logs "Job started on {name}"
- POST /jobs/{id}/stop: job → queued, clears printer_id + started_at,
  printer → idle, logs "Job stopped (was on {name})"
- POST /jobs/{id}/finish: job → done, sets finished_at + duration_minutes,
  printer → idle, logs "Job finished on {name}"
- Job detail shows Start / Stop+Finish buttons based on current status
- All printers currently printing are excluded from the Start printer picker

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

app/templates/job_detail.html

@@ -25,6 +25,29 @@
   </button>
 </div>
 
+{# ── Job actions ── #}
+{% set s = job.status.value %}
+{% if s == 'queued' %}
+<div class="mb-4">
+  <button class="btn btn-success btn-sm" data-bs-toggle="modal" data-bs-target="#startModal">
+    <i class="bi bi-play-fill me-1"></i>Start Job
+  </button>
+</div>
+{% elif s == 'printing' %}
+<div class="d-flex gap-2 mb-4">
+  <form method="POST" action="/jobs/{{ job.id }}/finish">
+    <button type="submit" class="btn btn-success btn-sm">
+      <i class="bi bi-check-lg me-1"></i>Finish
+    </button>
+  </form>
+  <form method="POST" action="/jobs/{{ job.id }}/stop">
+    <button type="submit" class="btn btn-outline-warning btn-sm">
+      <i class="bi bi-stop-fill me-1"></i>Stop
+    </button>
+  </form>
+</div>
+{% endif %}
+
 <div class="row g-3 mb-4">
 
   {# ── Job info ── #}
@@ -312,4 +335,41 @@
     </div>
   </div>
 </div>
+
+{# ── Start Job Modal ── #}
+{% if job.status.value == 'queued' %}
+<div class="modal fade" id="startModal" tabindex="-1">
+  <div class="modal-dialog">
+    <div class="modal-content bg-dark border-secondary">
+      <form method="POST" action="/jobs/{{ job.id }}/start">
+        <div class="modal-header border-secondary">
+          <h6 class="modal-title text-white">Start Job on Printer</h6>
+          <button type="button" class="btn-close btn-close-white" data-bs-dismiss="modal"></button>
+        </div>
+        <div class="modal-body">
+          {% if available_printers %}
+          <label class="form-label text-secondary small">Select printer <span class="text-danger">*</span></label>
+          <select name="printer_id" class="form-select bg-dark border-secondary text-white" required>
+            <option value="">— select —</option>
+            {% for p in available_printers %}
+            <option value="{{ p.id }}">{{ p.name }}{% if p.location %} · {{ p.location }}{% endif %} ({{ p.status.value }})</option>
+            {% endfor %}
+          </select>
+          {% else %}
+          <p class="text-warning mb-0">
+            <i class="bi bi-exclamation-triangle me-1"></i>All printers are currently busy.
+          </p>
+          {% endif %}
+        </div>
+        <div class="modal-footer border-secondary">
+          <button type="button" class="btn btn-secondary btn-sm" data-bs-dismiss="modal">Cancel</button>
+          <button type="submit" class="btn btn-success btn-sm" {% if not available_printers %}disabled{% endif %}>
+            <i class="bi bi-play-fill me-1"></i>Start
+          </button>
+        </div>
+      </form>
+    </div>
+  </div>
+</div>
+{% endif %}
 {% endblock %}