fix(cover): release the DB connection before the FTP thumbnail download (#2572)

GET /printers/{id}/cover took its printer row via Depends(get_db), whose
yield-dependency session stays open for the whole request — including the
3MF cover download (up to 8 remote paths x retries with backoff, minutes
under FTP contention). One pooled connection sat idle-in-transaction the
entire time; on a large farm a wall of dashboards drained the pool. The
route now fetches the printer in a short-lived async_session() and releases
the connection before the download (expire_on_commit=False keeps printer.*
readable). Pinned by a signature-inspection guard that fails if get_db is
ever re-added.

fix(print-start): release the DB connection across plate detection and 3MF download (#2572)

on_print_start held one session from top to bottom of the handler, across
two slow I/O blocks that need no database: the plate-detection camera grab
and, on the new-archive path, the multi-path 3MF FTP download (its own
comments cite worst cases of tens of minutes). The connection sat idle-in-
transaction for both, once per starting print. It now commits at each
boundary — only read SELECTs have run on those paths (every write branch
returns earlier), so the commit persists nothing and simply returns the
connection to the pool for the I/O; the next query re-acquires, and
expire_on_commit=False keeps printer.* readable.

fix(startup): connect to printers concurrently so the API serves within seconds (#2572)

init_printer_connections awaited each printer's connection serially, and
connect_printer ends in a fixed 1s settle wait. The MQTT connect is non-
blocking (connect_async + loop_start), so that 1s x fleet size was pure
serial dead air the FastAPI lifespan blocked on before uvicorn began
serving — ~100s before port 8000 responded on a 93-printer farm. The
connections are now started with asyncio.gather, so the step takes ~1s
regardless of fleet size. return_exceptions=True isolates each result: one
unreachable printer no longer aborts the rest, or startup itself.

backend/tests/unit/services/test_printer_manager.py

@@ -1804,6 +1804,37 @@ class TestInitPrinterConnections:
 
             mock_manager.connect_printer.assert_not_called()
 
+    @pytest.mark.asyncio
+    async def test_one_failing_printer_does_not_abort_the_rest(self):
+        """A single unreachable printer must not abort startup or the others (#2572).
+
+        The connections are gathered with return_exceptions=True. The old serial
+        ``await`` loop had no error handling, so the first printer that raised
+        propagated straight out of init_printer_connections and failed the
+        FastAPI lifespan — taking the whole app down over one bad row, and
+        skipping every printer after it. This asserts the isolation: every
+        printer is still attempted and the exception never escapes.
+        """
+        mock_db = AsyncMock()
+        printers = [MagicMock(id=i, name=f"p{i}", is_active=True) for i in range(3)]
+        mock_result = MagicMock()
+        mock_result.scalars.return_value.all.return_value = printers
+        mock_db.execute.return_value = mock_result
+
+        async def connect(printer):
+            if printer.id == 1:
+                raise ConnectionError("printer 1 is unreachable")
+            return True
+
+        with patch("backend.app.services.printer_manager.printer_manager") as mock_manager:
+            mock_manager.connect_printer = AsyncMock(side_effect=connect)
+
+            # Must not raise despite printer 1 failing.
+            await init_printer_connections(mock_db)
+
+            # All three were still attempted (not aborted at the failing one).
+            assert mock_manager.connect_printer.call_count == 3
+
 
 class TestAmsChangeCallback:
     """Tests for AMS change callback functionality."""