P2 cleanups: pin deps, prune job-run rows, unmask non-secret creds, enforce allowlist at login

- R11: pin requirements.txt to the versions running in the image, so a rebuild
  can't pull a breaking upstream release. Documented how to upgrade.
- R12: log rotation now also prunes job_runs rows older than 90 days (nulling
  the manual_imports FK reference first), so the table doesn't grow without
  bound. dedup/genre candidates are review state and left alone.
- U5: credential form shows non-secret fields (URLs, usernames, prefs) as plain
  text so they can be verified while typing; only real secrets stay masked.
  Dashboard IP card reworded from gluetun-specific to generic "Outbound IP".
- S9: enforce ALLOWED_EMAIL at the OIDC callback, before any user row or session
  is created, instead of only on later requests.

Verified: pinned build resolves; prune deletes only old rows and keeps the
manual_imports record; credentials page renders text+password inputs; a
disallowed email gets 403 with no user row, an allowed one succeeds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/templates/dashboard.html

@@ -39,12 +39,12 @@
   </div>
   <div class="stat-card">
     <div class="stat-value mono" style="font-size:1.5rem;"
-         title="{% if public_ip.error and not public_ip.ip %}Couldn't check: {{ public_ip.error }}{% elif public_ip.error %}Showing last known good check -- most recent check failed: {{ public_ip.error }}{% else %}This should be gluetun's VPN IP, not your home connection's{% endif %}">
+         title="{% if public_ip.error and not public_ip.ip %}Couldn't check: {{ public_ip.error }}{% elif public_ip.error %}Showing last known good check -- most recent check failed: {{ public_ip.error }}{% else %}alembic's outbound IP. If you route Soulseek through a VPN, confirm this is the VPN's IP and not your home connection's.{% endif %}">
       {% if public_ip.ip %}{{ public_ip.ip }}{% else %}<span class="muted">unknown</span>{% endif %}
     </div>
     <div class="stat-label">
       <span class="status-dot {{ 'status-dot-success' if public_ip.ip and not public_ip.error else ('status-dot-warning' if public_ip.ip else 'status-dot-muted') }}" style="margin-right:0.3rem;"></span>
-      Public IP <span class="muted">(via gluetun)</span>
+      Outbound IP
     </div>
   </div>
 </div>