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/db/database.py

@@ -12,12 +12,19 @@ def init_db() -> None:
 
     Idempotent — safe to call on every application startup.
     Creates all ORM tables if they do not already exist.
+
+    Closes any existing connection before re-initializing so that test
+    fixtures can monkeypatch DB_PATH between test runs.
     """
     from imptune.db.models import Client, Driver, Printer, Icon
 
     # Re-read DB_PATH each time so tests can patch imptune.config.DB_PATH
     import imptune.config as cfg
 
+    # Close any lingering connection from a previous run (important for tests)
+    if not db.is_closed():
+        db.close()
+
     db.init(
         cfg.DB_PATH,
         pragmas={