feat(session): per-owner printer/config storage via cookie-scoped bearer key

Printers and groups (Client) are now scoped to an Owner identified by an opaque
bearer key (secrets.token_urlsafe(32)) stored in an httponly cookie, defaulting
to temporary. First-visit modal offers backup-key download (marks permanent) or
temporary-only choice. /session/restore re-attaches a fresh browser to a saved
key. Every printer-facing route enforces ownership (404 on mismatch, not just
filtering) since printer IDs are sequential ints. Drivers stay global/shared.

On upgrade, pre-existing printer/client rows backfill to a synthetic legacy Owner;
its key is written to {DATA_DIR}/legacy_owner_key.txt for manual restore.

SECURITY: Added Origin/Referer same-origin check on POST /session/restore to
block login-CSRF/session-fixation attacks (cross-site form POST can't re-point
victim's cookie at attacker's Owner without hitting that check first).

Tests: 140 pass (2 deselected: pre-existing locale-flaky, unrelated to this change).
Verified live: modal on first visit, isolation between browsers, backup-key
download and restore flow work end-to-end.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

tests/test_packages.py

@@ -22,7 +22,7 @@ def driver_zip_bytes():
 
 
 @pytest.fixture
-def setup_printer_with_driver(tmp_data_dir, driver_zip_bytes):
+def setup_printer_with_driver(tmp_data_dir, driver_zip_bytes, owner):
     """Create Driver record (with ZIP on disk) and Printer record linked to it."""
     import hashlib
     import os
@@ -50,6 +50,7 @@ def setup_printer_with_driver(tmp_data_dir, driver_zip_bytes):
         ip_address="192.168.1.100",
         port_name="IP_192.168.1.100",
         driver=driver,
+        owner=owner,
         duplex_mode="OneSided",
         color_mode=True,
         paper_size="A4",
@@ -59,7 +60,7 @@ def setup_printer_with_driver(tmp_data_dir, driver_zip_bytes):
 
 
 @pytest.fixture
-def printer_no_driver(tmp_data_dir):
+def printer_no_driver(tmp_data_dir, owner):
     """Create Printer record with no driver assigned."""
     from imptune.db.models import Printer
 
@@ -68,6 +69,7 @@ def printer_no_driver(tmp_data_dir):
         ip_address="10.0.0.1",
         port_name="IP_10.0.0.1",
         driver=None,
+        owner=owner,
     )