fix(portal): per-challenge SSH user in web terminal + credential API

The web SSH terminal and the credential API reported `ctfuser` for all 16
challenges, but only the 6 native GEMASTIK XVIII images provision ctfuser.
Every imported XVI/XVII image does `RUN echo root:${PASSWORD} | chpasswd`,
so 10 of 16 participant logins were refused with "Permission denied".

Root causes (all the same class of bug - login hardcoded in the wrong layer):
- main.py websocket ssh handler read st["ssh_user"], a single team-wide value
  defaulting to ctfuser, instead of the per-challenge registry field
- /api/credential proxied the global receiver on :18080, which only knows the
  6 native challenges, so the other 10 returned "Invalid challenge"
- team.html hardcoded the challenge picker to those same 6 challenges, making
  the other 10 unreachable from the terminal entirely
- index.html rendered `<b>ctfuser</b>` and a stale hardcoded SSH port table

Fixes:
- orch.challenge_credential()/all_teams() read the TEAM's state.json, which
  holds the same per-challenge password the panel chpasswds
- gen_receiver_services.py injects SSH_USER_<port> from the registry so the
  receiver's /credential endpoint agrees with the panel
- receiver Challenge.credentials() honours SSH_USER_<port> (ctfuser fallback)
- new /api/team/{idx}/own-challenges feeds the picker; targets now carry
  challenge + ssh_user
- UI takes user and port from the server instead of hardcoding them

Verified: 32/32 credential payloads correct across teams 1-2, and 32/32 real
paramiko SSH logins succeed with whoami confirming the expected account.

Also adds bulk team delete: POST /api/teams/bulk-delete runs one background
thread and is polled via GET /api/teams/bulk-delete/{job_id}, plus per-team
checkboxes with select-all/clear in the UI. Deletion must stay sequential
because delete_team() regenerates shared artifacts at the end.

panel/static/guide.html

@@ -51,10 +51,10 @@
       <li>Buka <b>portal tim kamu</b> (domain dari panitia).</li>
       <li>Login dengan <b>password SSH tim</b>.</li>
       <li>Tab <b>🖥️ Terminal SSH</b> → pilih challenge → <b>Sambung</b>.</li>
-      <li>Langsung masuk sebagai <code>ctfuser</code> — tanpa perlu aplikasi SSH.</li>
+      <li>Langsung masuk sebagai user yang tertera di dropdown — <code>ctfuser</code> untuk 6 challenge native GEMASTIK XVIII, <code>root</code> untuk challenge XVI/XVII import — tanpa perlu aplikasi SSH.</li>
     </ol>
     <h3>Cara 2 — SSH Client (opsional)</h3>
-    <div class="sshbox">ssh ctfuser@43.134.105.109 -p &lt;PORT_SSH&gt;</div>
+    <div class="sshbox">ssh &lt;USER&gt;@43.134.105.109 -p &lt;PORT_SSH&gt;</div>
     <p>Contoh: untuk challenge <code>blogpost</code> tim 1, port SSH = <code>31022</code>.</p>
   </div>