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/main.py

@@ -5,9 +5,9 @@ from pathlib import Path
 from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 
-from imptune.api import drivers, health, pages
+from imptune.api import clients, drivers, health, pages, printers
 from imptune.config import DATA_DIR, DRIVERS_DIR
-from imptune.db.database import init_db
+from imptune.db.database import db, init_db
 
 
 @asynccontextmanager
@@ -16,6 +16,9 @@ async def lifespan(app: FastAPI):
     os.makedirs(DRIVERS_DIR, exist_ok=True)
     init_db()
     yield
+    # Close DB connection on shutdown so test fixtures can re-initialize cleanly
+    if not db.is_closed():
+        db.close()
 
 
 app = FastAPI(title="ImpTune", lifespan=lifespan)
@@ -28,3 +31,5 @@ app.mount("/static", StaticFiles(directory=str(_static_dir)), name="static")
 app.include_router(health.router)
 app.include_router(pages.router)
 app.include_router(drivers.router)
+app.include_router(printers.router)
+app.include_router(clients.router)