fix(vp): slice FTP passive ports per VP, drop bridge-mode RAM by 95% (#1646)

The 50000-51000 docker-compose port range spawned ~2000 docker-proxy
  host processes (~3.5 GB RSS) under Docker's default userland-proxy.
  The 1001-port pool was symptom treatment — collisions only matter for
  multi-VP-on-shared-bind, but the cost was paid by every install.

  Each VP now gets a non-overlapping 10-port slice computed from its id
  (VP 1 -> 50000-50009, VP 2 -> 50010-50019, ...). Class constants are
  gone; VirtualPrinterFTPServer takes passive_port_min/max instance args.
  Wraps modulo PASSIVE_MAX_SLOTS = 100, with the existing 10-attempt
  random retry as same-slot collision fallback.

  Compose default narrowed to 50000-50029 (3 VPs). Proxy-mode VPs forward
  the real printer's full range and stay on the separate TCPProxy
  constants. Compose comment rewritten to acknowledge Linux multi-service
  hosts as a primary bridge-mode audience and drop an over-stated
  "confirmed by reporter" claim about userland-proxy=false.

backend/tests/unit/services/test_vp_ftp_port_slicing.py

@@ -0,0 +1,133 @@
+"""Tests for the per-VP FTP passive-port slice helper (#1646).
+
+Each VP is allocated a non-overlapping 10-port slice from the
+PASSIVE_PORT_BASE pool. Bridge-mode Docker users only have to expose
+`PASSIVE_SLICE_SIZE * N_vps` ports instead of the historical 1001-port
+pool that spawned ~2000 docker-proxy host processes (~3.5 GB RAM).
+
+Slicing properties pinned here:
+  - Slice 0 covers PASSIVE_PORT_BASE..+SLICE_SIZE-1 (the only slice that
+    aligns with the compose file's narrowest default exposure).
+  - Each subsequent vp_id advances by exactly SLICE_SIZE — no overlap, no
+    gap.
+  - vp_ids beyond MAX_SLOTS wrap around (modulo) so installs that have
+    churned through many VPs over time still produce a valid in-range
+    slice; same-slot collisions fall back to the per-session 10-attempt
+    random retry, which is the same behaviour as pre-#1646.
+  - Defensive: a non-positive vp_id (shouldn't occur, but DBs are
+    surprising) clamps to slot 0 rather than producing a negative port.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from backend.app.services.virtual_printer.ftp_server import (
+    PASSIVE_MAX_SLOTS,
+    PASSIVE_PORT_BASE,
+    PASSIVE_SLICE_SIZE,
+    compute_passive_port_slice,
+)
+
+
+class TestComputePassivePortSlice:
+    def test_vp_id_one_starts_at_base(self):
+        port_min, port_max = compute_passive_port_slice(1)
+        assert port_min == PASSIVE_PORT_BASE
+        assert port_max == PASSIVE_PORT_BASE + PASSIVE_SLICE_SIZE - 1
+
+    def test_consecutive_vp_ids_get_adjacent_non_overlapping_slices(self):
+        slice1_min, slice1_max = compute_passive_port_slice(1)
+        slice2_min, slice2_max = compute_passive_port_slice(2)
+        slice3_min, slice3_max = compute_passive_port_slice(3)
+        assert slice1_max + 1 == slice2_min  # no gap
+        assert slice2_max + 1 == slice3_min  # no gap
+        # Slice width matches the constant — no off-by-one.
+        assert slice1_max - slice1_min + 1 == PASSIVE_SLICE_SIZE
+        assert slice2_max - slice2_min + 1 == PASSIVE_SLICE_SIZE
+        assert slice3_max - slice3_min + 1 == PASSIVE_SLICE_SIZE
+
+    def test_no_two_distinct_vp_ids_within_max_slots_share_a_port(self):
+        seen: dict[int, int] = {}
+        for vp_id in range(1, PASSIVE_MAX_SLOTS + 1):
+            lo, hi = compute_passive_port_slice(vp_id)
+            for port in range(lo, hi + 1):
+                assert port not in seen, f"VP {vp_id} clashes with VP {seen[port]} on port {port}"
+                seen[port] = vp_id
+
+    def test_wraps_modulo_max_slots(self):
+        """VP id past MAX_SLOTS lands on the same slice as its modulo-N
+        neighbour — the per-session retry handles the rare collision."""
+        first = compute_passive_port_slice(1)
+        wrapped = compute_passive_port_slice(PASSIVE_MAX_SLOTS + 1)
+        assert wrapped == first
+
+    def test_top_slot_is_within_base_pool(self):
+        """The last valid slot must stay below PASSIVE_PORT_BASE +
+        MAX_SLOTS*SLICE_SIZE so the slice never escapes the pool that
+        the docker-compose comments document for users."""
+        _, hi = compute_passive_port_slice(PASSIVE_MAX_SLOTS)
+        assert hi < PASSIVE_PORT_BASE + PASSIVE_MAX_SLOTS * PASSIVE_SLICE_SIZE
+
+    @pytest.mark.parametrize("bad_id", [0, -1, -999])
+    def test_non_positive_ids_clamp_to_slot_zero(self, bad_id):
+        """Defensive: a bad vp_id from a corrupted row mustn't produce a
+        negative port and crash asyncio.start_server."""
+        assert compute_passive_port_slice(bad_id) == compute_passive_port_slice(1)
+
+
+class TestFTPServerHonoursPerInstanceRange:
+    """`VirtualPrinterFTPServer` now stores the slice on `self`. Two
+    instances constructed with different slices must hand each
+    `FTPSession` the right per-instance range — no class-level leak."""
+
+    def test_two_instances_independent_ranges(self, tmp_path):
+        from backend.app.services.virtual_printer.ftp_server import VirtualPrinterFTPServer
+
+        cert = tmp_path / "cert.pem"
+        cert.write_text("not a real cert")
+        key = tmp_path / "key.pem"
+        key.write_text("not a real key")
+
+        a = VirtualPrinterFTPServer(
+            upload_dir=tmp_path,
+            access_code="x",
+            cert_path=cert,
+            key_path=key,
+            passive_port_min=50000,
+            passive_port_max=50009,
+        )
+        b = VirtualPrinterFTPServer(
+            upload_dir=tmp_path,
+            access_code="x",
+            cert_path=cert,
+            key_path=key,
+            passive_port_min=50050,
+            passive_port_max=50059,
+        )
+
+        assert (a.passive_port_min, a.passive_port_max) == (50000, 50009)
+        assert (b.passive_port_min, b.passive_port_max) == (50050, 50059)
+        # Mutating one must not affect the other (regression guard against
+        # the pre-fix class-constant layout).
+        b.passive_port_min = 60000
+        assert a.passive_port_min == 50000
+
+    def test_default_construction_gives_a_one_slice_window(self, tmp_path):
+        """A consumer that doesn't pass passive_port_min/max should still
+        get a valid, minimal range — handy for tests and direct callers."""
+        from backend.app.services.virtual_printer.ftp_server import VirtualPrinterFTPServer
+
+        cert = tmp_path / "cert.pem"
+        cert.write_text("x")
+        key = tmp_path / "key.pem"
+        key.write_text("x")
+
+        server = VirtualPrinterFTPServer(
+            upload_dir=tmp_path,
+            access_code="x",
+            cert_path=cert,
+            key_path=key,
+        )
+        assert server.passive_port_min == PASSIVE_PORT_BASE
+        assert server.passive_port_max - server.passive_port_min + 1 == PASSIVE_SLICE_SIZE