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/templates/session_restore.html

@@ -0,0 +1,20 @@
+{% extends "base.html" %}
+
+{% block content %}
+<h1>Restore backup key</h1>
+
+<p>Paste the key from your <code>imptune-backup-key.txt</code> backup file to
+recover your printers, configs, and groups on this browser.</p>
+
+{% if error %}
+<div class="error"><p>{{ error }}</p></div>
+{% endif %}
+
+<form method="post" action="/session/restore">
+  <label>
+    Backup key
+    <input type="text" name="key" placeholder="paste your key here" required autofocus>
+  </label>
+  <button type="submit">Restore</button>
+</form>
+{% endblock %}