fix(creds): MQTT regression + secret_kind for hash credentials

Honest correction to the "every cred-emitting service" claim. Audit
of templates/* found three gaps:

1. MQTT — was working through the legacy adapter, silently dropped
   when Phase 3 (e696c2b) deleted it. Now migrated to encode_secret()
   alongside the others.
2. Postgres — `auth, pw_hash=…` event captures the MD5
   challenge-response the attacker sent. Plaintext irrecoverable, so
   it never fit the (principal, secret_b64=raw_bytes) shape. Lands
   in Credential as secret_kind="postgres_md5_challenge".
3. VNC — `auth_response, response=…hex` event captures the 16-byte
   DES-encrypted challenge. Same situation as Postgres: plaintext
   irrecoverable. Lands as secret_kind="vnc_des_response".

Adds a `secret_kind` discriminator column to Credential (default
"plaintext", indexed). The dedup tuple gains secret_kind so two
credentials with the same sha256 but different kinds are
fundamentally different rows — different challenges produce
different bytes for the same plaintext password, so cross-kind
reuse matches are meaningless and would only confuse analytics.

The model now genuinely covers every cred-emitting service in the
fleet:

  plaintext        SSH, Telnet, FTP, POP3, IMAP, SMTP, Redis, LDAP,
                   MQTT
  postgres_md5_*   Postgres
  vnc_des_response VNC

Username-only services (MySQL/MSSQL — TDS pre-encryption captures
the user but never sees the password byte) intentionally don't feed
Credential — they're recon signals, not cred attempts.

40 tests pass in the touched scope. New cases: secret_kind dedups
independently in the repo; Postgres MD5 + VNC DES emitters thread
through; MQTT round-trips through the native branch.

decnet/web/ingester.py

@@ -246,12 +246,14 @@ async def _ingest_credential_native(
     sha256_hex = hashlib.sha256(raw).hexdigest()
     principal = fields.get("principal") or fields.get("username")
     secret_printable = fields.get("secret_printable")
+    secret_kind = fields.get("secret_kind") or "plaintext"
 
     await repo.upsert_credential({
         "attacker_ip": log_data.get("attacker_ip"),
         "decky_name": log_data.get("decky"),
         "service": log_data.get("service"),
         "principal": _truncate_with_warn(principal, _PRINCIPAL_MAX, "principal"),
+        "secret_kind": secret_kind,
         "secret_sha256": sha256_hex,
         "secret_b64": _truncate_with_warn(b64, _SECRET_B64_MAX, "secret_b64"),
         "secret_printable": _truncate_with_warn(