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.

development/DEBT.md

@@ -1,6 +1,6 @@
 # DECNET — Technical Debt Register
 
-> Last updated: 2026-04-25 — DEBT-039 resolved (six service emitters on standardized shape, legacy ingester adapter deleted).
+> Last updated: 2026-04-25 — Credential model gained `secret_kind` discriminator; Postgres MD5 + VNC DES challenge creds now land in the table; MQTT regression from the legacy-adapter removal patched.
 > Severity: 🔴 Critical · 🟠 High · 🟡 Medium · 🟢 Low
 
 ---