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/e2e/test_printer_edit.py

@@ -4,12 +4,16 @@ from __future__ import annotations
 import pytest
 
 
-def test_printer_edit_modal_open_and_prefill(page, live_server: str) -> None:
+def test_printer_edit_modal_open_and_prefill(page, live_server: str, _e2e_owner_key: str) -> None:
     """Edit button opens modal with printer's current name pre-filled."""
     import httpx
 
-    # Create a printer via API
-    with httpx.Client(base_url=live_server, follow_redirects=True) as api:
+    from imptune.services.session import COOKIE_NAME
+
+    # Create a printer via API, under the same owner the page's cookie is seeded with
+    with httpx.Client(
+        base_url=live_server, follow_redirects=True, cookies={COOKIE_NAME: _e2e_owner_key}
+    ) as api:
         api.post(
             "/printers",
             data={
@@ -30,11 +34,15 @@ def test_printer_edit_modal_open_and_prefill(page, live_server: str) -> None:
     assert name_val == "EditTest Printer"
 
 
-def test_printer_edit_submit_updates_list(page, live_server: str) -> None:
+def test_printer_edit_submit_updates_list(page, live_server: str, _e2e_owner_key: str) -> None:
     """Submitting the edit form updates the printer name in the list (no page reload)."""
     import httpx
 
-    with httpx.Client(base_url=live_server, follow_redirects=True) as api:
+    from imptune.services.session import COOKIE_NAME
+
+    with httpx.Client(
+        base_url=live_server, follow_redirects=True, cookies={COOKIE_NAME: _e2e_owner_key}
+    ) as api:
         resp = api.post(
             "/printers",
             data={