fix(security): remove decrypt oracle, redact secret logs, harden auth, bump Go

Address multiple vulnerabilities found during security review:

- Remove unauthenticated POST /api/crypto/decrypt decryption oracle (route,
  handler, dead frontend helper) + regression test. Transport encryption is
  one-directional; the server never needs to decrypt arbitrary client payloads.
- Redact secrets in config-update logs: handler_ai_model/handler_exchange logged
  %+v of decrypted requests, leaking API keys / secret keys / passphrases /
  private keys. Use named types shared with the log sanitizer so the masking
  can never drift again; extend masking to passphrase + lighter_api_key_private_key.
- crypto: require a valid timestamp in DecryptPayload (a missing ts previously
  skipped replay protection entirely).
- crypto: EncryptedString.Value() now fails closed instead of silently
  persisting plaintext secrets when encryption errors.
- auth: per-IP token-bucket rate limiting on /login and /register against online
  brute-force; raise registration password minimum 6 -> 8; add dummy bcrypt
  compare on unknown-email login to close the user-enumeration timing channel.
- IDOR: getTraderFromQuery no longer falls back to the global in-memory trader
  map; trader access is strictly scoped to the authenticated caller.
- Bump Go 1.25.10 -> 1.25.11 to resolve reachable net/textproto and crypto/x509
  stdlib advisories (govulncheck now reports 0 affecting vulnerabilities).

crypto/crypto.go

@@ -282,12 +282,16 @@ func isEncryptedStorageValue(value string) bool {
 }
 
 func (cs *CryptoService) DecryptPayload(payload *EncryptedPayload) ([]byte, error) {
-	// 1. Validate timestamp (prevent replay attacks)
-	if payload.TS != 0 {
-		elapsed := time.Since(time.Unix(payload.TS, 0))
-		if elapsed > 5*time.Minute || elapsed < -1*time.Minute {
-			return nil, errors.New("timestamp invalid or expired")
-		}
+	// 1. Validate timestamp (prevent replay attacks).
+	// The timestamp is mandatory: a missing/zero ts previously skipped this check
+	// entirely, which let a captured ciphertext be replayed indefinitely. The
+	// client (web/src/lib/crypto.ts) always stamps ts, so requiring it is safe.
+	if payload.TS == 0 {
+		return nil, errors.New("missing timestamp")
+	}
+	elapsed := time.Since(time.Unix(payload.TS, 0))
+	if elapsed > 5*time.Minute || elapsed < -1*time.Minute {
+		return nil, errors.New("timestamp invalid or expired")
 	}
 
 	// 2. Decode base64url
@@ -455,8 +459,11 @@ func (es EncryptedString) Value() (driver.Value, error) {
 	if globalCryptoService != nil {
 		encrypted, err := globalCryptoService.EncryptForStorage(string(es))
 		if err != nil {
-			// If encryption fails, return the original value
-			return string(es), nil
+			// Fail closed: never silently persist a plaintext secret when
+			// encryption was expected to happen. Returning the error aborts the
+			// write so a misconfigured/broken crypto service cannot leak
+			// credentials into the database in cleartext.
+			return nil, fmt.Errorf("failed to encrypt sensitive field for storage: %w", err)
 		}
 		return encrypted, nil
 	}