Add web UI for SSH CA, Transit, and User engines; full security audit and remediation

Web UI: Added browser-based management for all three remaining engines
(SSH CA, Transit, User E2E). Includes gRPC client wiring, handler files,
7 HTML templates, dashboard mount forms, and conditional navigation links.
Fixed REST API routes to match design specs (SSH CA cert singular paths,
Transit PATCH for update-key-config).

Security audit: Conducted full-system audit covering crypto core, all
engine implementations, API servers, policy engine, auth, deployment,
and documentation. Identified 42 new findings (#39-#80) across all
severity levels.

Remediation of all 8 High findings:
- #68: Replaced 14 JSON-injection-vulnerable error responses with safe
  json.Encoder via writeJSONError helper
- #48: Added two-layer path traversal defense (barrier validatePath
  rejects ".." segments; engine ValidateName enforces safe name pattern)
- #39: Extended RLock through entire crypto operations in barrier
  Get/Put/Delete/List to eliminate TOCTOU race with Seal
- #40: Unified ReWrapKeys and seal_config UPDATE into single SQLite
  transaction to prevent irrecoverable data loss on crash during MEK
  rotation
- #49: Added resolveTTL to CA engine enforcing issuer MaxTTL ceiling
  on handleIssue and handleSignCSR
- #61: Store raw ECDH private key bytes in userState for effective
  zeroization on Seal
- #62: Fixed user engine policy resource path from mountPath to
  mountName() so policy rules match correctly
- #69: Added newPolicyChecker helper and passed service-level policy
  evaluation to all 25 typed REST handler engine.Request structs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/engine/ca/ca.go

@@ -634,6 +634,9 @@ func (e *CAEngine) handleCreateIssuer(ctx context.Context, req *engine.Request)
 	if name == "" {
 		return nil, fmt.Errorf("ca: issuer name is required")
 	}
+	if err := engine.ValidateName(name); err != nil {
+		return nil, fmt.Errorf("ca: %w", err)
+	}
 
 	e.mu.Lock()
 	defer e.mu.Unlock()
@@ -873,10 +876,13 @@ func (e *CAEngine) handleIssue(ctx context.Context, req *engine.Request) (*engin
 		return nil, fmt.Errorf("%w: %s", ErrUnknownProfile, profileName)
 	}
 
-	// Apply user overrides.
-	if v, ok := req.Data["ttl"].(string); ok && v != "" {
-		profile.Expiry = v
+	// Validate and apply TTL against issuer MaxTTL.
+	requestedTTL, _ := req.Data["ttl"].(string)
+	ttl, err := resolveTTL(requestedTTL, is.config.MaxTTL)
+	if err != nil {
+		return nil, err
 	}
+	profile.Expiry = ttl.String()
 	if v, ok := req.Data["key_usages"].([]interface{}); ok {
 		profile.KeyUse = toStringSlice(v)
 	}
@@ -1259,10 +1265,6 @@ func (e *CAEngine) handleSignCSR(ctx context.Context, req *engine.Request) (*eng
 		return nil, fmt.Errorf("%w: %s", ErrUnknownProfile, profileName)
 	}
 
-	if v, ok := req.Data["ttl"].(string); ok && v != "" {
-		profile.Expiry = v
-	}
-
 	e.mu.Lock()
 	defer e.mu.Unlock()
 
@@ -1275,6 +1277,14 @@ func (e *CAEngine) handleSignCSR(ctx context.Context, req *engine.Request) (*eng
 		return nil, ErrIssuerNotFound
 	}
 
+	// Validate and apply TTL against issuer MaxTTL.
+	requestedTTL, _ := req.Data["ttl"].(string)
+	ttl, err := resolveTTL(requestedTTL, is.config.MaxTTL)
+	if err != nil {
+		return nil, err
+	}
+	profile.Expiry = ttl.String()
+
 	// Authorization: admins bypass; otherwise check identifiers from the CSR.
 	if !req.CallerInfo.IsAdmin {
 		sans := append(csr.DNSNames, ipStrings(csr.IPAddresses)...)
@@ -1497,6 +1507,25 @@ func zeroizeKey(key crypto.PrivateKey) {
 	}
 }
 
+// resolveTTL parses and validates a requested TTL against the issuer's MaxTTL.
+func resolveTTL(requested, issuerMaxTTL string) (time.Duration, error) {
+	maxTTL, err := time.ParseDuration(issuerMaxTTL)
+	if err != nil || maxTTL <= 0 {
+		maxTTL = 2160 * time.Hour // 90 days fallback
+	}
+	if requested != "" {
+		ttl, err := time.ParseDuration(requested)
+		if err != nil {
+			return 0, fmt.Errorf("ca: invalid TTL %q: %w", requested, err)
+		}
+		if ttl > maxTTL {
+			return 0, fmt.Errorf("ca: requested TTL %s exceeds issuer maximum %s", ttl, maxTTL)
+		}
+		return ttl, nil
+	}
+	return maxTTL, nil
+}
+
 func toStringSlice(v []interface{}) []string {
 	s := make([]string, 0, len(v))
 	for _, item := range v {