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_static.py

@@ -53,7 +53,7 @@ def test_theme_toggle_present(client):
     assert "cycle" in response.text or "store.theme" in response.text
 
 
-def test_dashboard_shows_recent_printers(client):
+def test_dashboard_shows_recent_printers(client, owner):
     """Dashboard renders names of recently-created printers from DB."""
     from imptune.db.models import Printer
 
@@ -61,11 +61,13 @@ def test_dashboard_shows_recent_printers(client):
         name="TestPrinter-Alpha",
         ip_address="10.0.0.1",
         port_name="IP_10.0.0.1",
+        owner=owner,
     )
     Printer.create(
         name="TestPrinter-Beta",
         ip_address="10.0.0.2",
         port_name="IP_10.0.0.2",
+        owner=owner,
     )
 
     response = client.get("/")
@@ -85,7 +87,7 @@ def test_theme_toggle_present(client):
     assert "cycle" in response.text or "store.theme" in response.text
 
 
-def test_dashboard_shows_recent_packages(client):
+def test_dashboard_shows_recent_packages(client, owner):
     """Dashboard recent-packages section shows only printers with a driver assigned."""
     from imptune.db.models import Driver, Printer
 
@@ -100,11 +102,13 @@ def test_dashboard_shows_recent_packages(client):
         ip_address="10.0.0.2",
         port_name="IP_10.0.0.2",
         driver=driver,
+        owner=owner,
     )
     Printer.create(
         name="PkgPrinter-NoDriver",
         ip_address="10.0.0.3",
         port_name="IP_10.0.0.3",
+        owner=owner,
     )
 
     response = client.get("/")