feat(03-01): implement printer and client CRUD with HTMX/Alpine.js UI

- imptune/api/printers.py: POST /printers (all form fields, checkbox->bool,
  FK resolution), DELETE /printers/{id}, grouped list renderer with LEFT JOIN
- imptune/api/clients.py: POST /clients with duplicate-name handling
- imptune/api/pages.py: GET /printers and GET /clients page routes
- imptune/main.py: register printers + clients routers; close db on shutdown
- imptune/db/database.py: close existing connection before re-init (test isolation)
- templates: printers.html, clients.html, printer_form.html (Alpine.js port
  auto-derivation), printer_list.html (grouped by client), client_list.html
- tests/conftest.py: close test-thread db connection in fixture teardown
- tests/test_printer_crud.py: updated to use list(select().where()) for DB
  queries (avoids Peewee thread-local cursor caching across test boundaries)

Auto-fix [Rule 1 - Bug]: DB test isolation — Peewee thread-local connections
persisted across tests causing stale DB reads; fixed via conftest teardown and
lifespan db.close() on shutdown.

imptune/templates/partials/client_list.html

@@ -0,0 +1,22 @@
+<div id="client-list">
+  {% if not clients %}
+  <p>No clients configured yet.</p>
+  {% else %}
+  <table>
+    <thead>
+      <tr>
+        <th>Name</th>
+        <th>Created</th>
+      </tr>
+    </thead>
+    <tbody>
+      {% for c in clients %}
+      <tr>
+        <td>{{ c.name }}</td>
+        <td>{{ c.created_at.strftime('%Y-%m-%d') if c.created_at else '—' }}</td>
+      </tr>
+      {% endfor %}
+    </tbody>
+  </table>
+  {% endif %}
+</div>