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>

imptune/config.py

@@ -7,6 +7,8 @@ load_dotenv()
 
 DATA_DIR = os.environ.get("DATA_DIR", "/data")
 PORT = int(os.environ.get("PORT", "8000"))
+# Set to "false" for local plain-HTTP dev — browsers drop Secure cookies over HTTP.
+COOKIE_SECURE = os.environ.get("COOKIE_SECURE", "true").lower() != "false"
 
 DB_PATH = str(Path(DATA_DIR) / "imptune.db")
 DRIVERS_DIR = str(Path(DATA_DIR) / "drivers")