fix(vp): overlay storage indicators on cached push so slicer pre-flight passes for P1S/A1 targets (issue #1228)

Slicer "Send to printer" worked on 0.2.3.2 with a queue-mode VP and
  started failing on 0.2.4b3 with BambuStudio's generic "storage needs
  to be inserted before send to printer" error. Multiple users
  reported it across P1S, P2S, Docker bridge, macvlan, and host
  networking. @rtadams89's debug-level support archive showed the
  smoking gun: slicer establishes MQTT TLS, gets pushall +
  get_version, then never opens an FTP connection — pre-flight
  rejects before any data transfer.

  The 0.2.3.2 synthetic stub baked in three SD/storage indicators
  that BambuStudio's "Send" pre-flight reads: home_flag with bit 8
  (HAS_SDCARD_NORMAL, 0x100), sdcard=True, and a storage:{free,total}
  block. The 0.2.4b3 cached-as-base slicer-mirror (7dea33d0) passes
  the live target's push_status through with only an IP rewrite — if
  the real firmware doesn't report those fields (P1S/A1 with no SD
  card, older field shapes, confirmed on P1S firmware 01.10.00.00),
  the slicer sees "no storage" and aborts. H2D and X1C reproductions
  worked because those firmwares do report the indicators.

  In _send_status_report's cached-as-base branch, after copying the
  cache and applying the existing protocol/upload-state overrides:

  - home_flag |= 0x100 (preserves any other bits the real printer set)
  - sdcard = True (force-set even when real says False)
  - storage = setdefault(...) (only fills in if missing — real values
    pass through unchanged when the printer reports them)

  For VP usage the slicer uploads via FTPS to Bambuddy's filesystem
  at /app/data/virtual_printer/uploads/<vpid>/; the printer's actual
  SD card is irrelevant on that path, so forcing "storage available"
  is correct for the queue / immediate / review modes the
  cached-as-base path covers.

backend/app/services/virtual_printer/mqtt_server.py

@@ -659,6 +659,20 @@ class SimpleMQTTServer:
                 else:
                     # Don't override real subtask_name with empty if no upload pending.
                     print_block.setdefault("subtask_name", "")
+                # Storage-availability indicators the slicer's "Send" pre-flight reads
+                # (#1228). P1S/A1-class firmware doesn't always include these in
+                # push_status (no SD card inserted, older field shapes), and BambuStudio
+                # rejects the send pre-flight with the generic "storage needs to be
+                # inserted before send to printer" error before even attempting FTP.
+                # For VP usage the slicer uploads via FTPS to Bambuddy's filesystem —
+                # the printer's actual SD/storage state is irrelevant on that path.
+                # Force "available" indicators so the pre-flight passes regardless of
+                # what the real printer reports. Restores the 0.2.3.2 synthetic-stub
+                # behaviour for these fields without losing the live AMS / k-profile /
+                # camera mirror cached-as-base provides.
+                print_block["home_flag"] = print_block.get("home_flag", 0) | 0x100  # bit 8 = HAS_SDCARD_NORMAL
+                print_block["sdcard"] = True
+                print_block.setdefault("storage", {"free": 1_000_000_000, "total": 32_000_000_000})
                 status = {"print": print_block}
                 await self._publish_to_report(writer, status, serial or self.serial)
                 return