fix(queue): skip preheat entirely when no loaded filament wants a chamber (issue #3041)

Preheat & Heat Soak delayed every PLA print by five to seven minutes and
gave nothing back. The filament map correctly derived a chamber target of
0, and the chamber phase correctly skipped -- but the stage then heated
the bed, waited for it, and held the full soak anyway, because the soak
had no idea it was holding for a chamber nobody asked for. The print's
own G-code sets the bed the moment it starts, so the bed phase only moved
the warm-up ahead of the FTP upload instead of overlapping with it.

A 0 that comes out of the filament map now skips the stage before any
command goes out. The one thing the skip still does is put the airduct
flap back to cooling on the models that have one -- an H2D left in
heating mode by the ABS job before it would otherwise cook the PLA that
follows, and that costs one MQTT command and no waiting.

Explicit instructions are untouched. A chamber target of 0 typed into a
print's own override still heats the bed and runs the soak, which is what
the queue documentation has always promised it does, as does forcing a
print's Preheat override to On. Prints that want chamber heat are
unaffected, including the P1S/P1P/A1 tier where the bed and the soak
timer are the whole mechanism.

The existing unit tests all ran with soak_seconds=0, which is why the
production default was never exercised; the PLA test now runs at the real
default and asserts nothing is dispatched and nothing is slept.

Surfaced in the UI on the way through: the Settings hint claimed the
derived 0 skipped "the chamber phase", and the per-print chamber override
field said nothing about a typed 0 meaning bed-only -- a user reaching
for 0 to turn preheat off got the delay instead.

backend/app/services/print_scheduler.py

@@ -5117,6 +5117,34 @@ class PrintScheduler:
                 return False
         return True
 
+    def _preheat_flap_to_cooling(self, item_id: int, printer: Printer) -> None:
+        """Put the airduct flap back to cooling for a print that wants no chamber heat.
+
+        The full preheat stage does this as part of its own dispatch: an H2D
+        left in heating mode by the ABS job before it would otherwise cook the
+        PLA that follows. The skip path never reaches that code, so it calls
+        this instead -- one idempotent MQTT command, no waiting, and nothing to
+        add to the rollback pin, because a flap set to cooling for a print that
+        needs no heat is where it should have been either way.
+
+        Best-effort like everything else in the stage: a refused command logs
+        and the dispatch carries on.
+        """
+        model = printer.model or ""
+        if not supports_airduct(model):
+            return
+        state = printer_manager.get_status(printer.id)
+        current = getattr(state, "airduct_mode", None) if state else None
+        if current == _AIRDUCT_MODE_COOLING:
+            return
+        client = printer_manager.get_client(printer.id)
+        if client is None:
+            return
+        try:
+            client.set_airduct_mode("cooling")
+        except Exception as exc:
+            logger.warning("Queue item %s: preheat-skip airduct cooling failed: %s", item_id, exc)
+
     async def _preheat_and_soak(
         self,
         db: AsyncSession,
@@ -5140,9 +5168,14 @@ class PrintScheduler:
           2. Chamber target — `item.preheat_chamber_target_override` if non-null;
              else max of `preheat_filament_targets[normalize(t.tray_type)]`
              across the trays `item.ams_mapping` names (every loaded slot when
-             it names none); else 0 (skips chamber phase, keeps bed phase +
-             soak timer).
-          3. Three hardware tiers branch the wait loop:
+             it names none).
+          3. A target of 0 off the filament map skips the whole stage: the
+             materials this print loads want no chamber, so there is nothing to
+             soak for and the bed phase would only delay the upload (#3041).
+             An explicit 0 typed into the per-item override, or a per-item
+             'on', still runs the bed phase and the soak — both are the user
+             asking for a warm bed in so many words.
+          4. Three hardware tiers branch the wait loop:
              - Chamber heater (H2C/H2D/H2DPro/H2S/X2D/X1E via supports_chamber_heater):
                send M141 to the resolved target, then wait for the chamber sensor
                to reach it (or the max-wait timeout to elapse).
@@ -5178,9 +5211,10 @@ class PrintScheduler:
         # Chamber target resolution:
         #   1. Explicit per-item override beats everything (user knows best).
         #   2. Otherwise derive from the filament types this print loads, via
-        #      the per-filament target map. PLA-only print derives 0 → chamber
-        #      phase auto-skips without the user touching anything, even when
-        #      an ASA spool is sitting in another slot of the same AMS (#2886).
+        #      the per-filament target map. A PLA-only print derives 0 and the
+        #      block below skips the stage without the user touching anything,
+        #      even when an ASA spool is sitting in another slot of the same
+        #      AMS (#2886).
         explicit_target = getattr(item, "preheat_chamber_target_override", None)
         if explicit_target is not None and explicit_target > 0:
             chamber_target = int(explicit_target)
@@ -5193,6 +5227,31 @@ class PrintScheduler:
             chamber_target = self._derive_chamber_target(printer, targets, item)
             chamber_source = "filament-map"
 
+        # Nothing to preheat *for*. A zero that came out of the filament map is
+        # the map saying this print's materials want no chamber conditioning --
+        # PLA, PETG, TPU and PVA all sit at 0 by default. Running the stage
+        # anyway heated the bed and then held it for the full soak, which
+        # delayed every PLA dispatch by minutes and bought nothing: the print's
+        # own G-code sets the bed the moment it starts, so preheating it here
+        # only moves that heating ahead of the upload instead of overlapping
+        # with it, and the soak has no chamber to condition (#3041).
+        #
+        # An explicit statement from the user still runs the stage. Forcing the
+        # per-item override to 'on', or typing a chamber target of exactly 0,
+        # both mean "preheat the bed for this print" -- the second is
+        # documented as doing precisely that. Only the automatic path, the
+        # global toggle plus the filament map, short-circuits here.
+        if chamber_target <= 0 and chamber_source == "filament-map" and override != "on":
+            logger.info(
+                "Queue item %s: preheat skipped -- the loaded filaments derive no chamber "
+                "target, so there is nothing to soak for (override=%s model=%s)",
+                item.id,
+                override,
+                printer.model or "",
+            )
+            self._preheat_flap_to_cooling(item.id, printer)
+            return True
+
         bed_target = int(archive.bed_temperature) if archive and archive.bed_temperature else 0
         if bed_target <= 0:
             # No bed temperature in the slicer metadata. When the print needs a