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/gen_receiver_services.py

@@ -33,6 +33,12 @@ def write_unit(idx: int, st: dict):
     # normalize the name to underscores here. main.py looks up the same key.
     # Same normalization applies to CHALLENGE_SCHEME_<NAME>.
     schemes = {c["name"]: c.get("scheme") for c in load_registry().get("challenges", [])}
+    # SSH login user per challenge. The panel already chpasswds the right
+    # account from this field (teams.challenge_ssh_users); the receiver must
+    # report the SAME user via /credential/<name> or participants get a login
+    # that cannot work. Registry is the single source of truth for both sides.
+    ssh_users = {c["name"]: c.get("ssh_user", "ctfuser")
+                 for c in load_registry().get("challenges", [])}
     for ch in st["ports"]:
         if ch in ("receiver", "panel"):
             continue
@@ -41,6 +47,8 @@ def write_unit(idx: int, st: dict):
         env[f"CHALLENGE_CONTAINER_{key}"] = f"{ch}_container_team{idx}"
         if schemes.get(ch):
             env[f"CHALLENGE_SCHEME_{key}"] = schemes[ch]
+        if ssh_users.get(ch):
+            env[f"SSH_USER_{st['ports'][ch]['chall']}"] = ssh_users[ch]
         # SSH passwords: checker Challenge.credentials() reads PASSWORD_<self.port>
         # where self.port is the team challenge port.
         pwd = st.get("chall_passwords", {}).get(ch)