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>

web/templates/layout.html

@@ -14,6 +14,9 @@
             {{if .Username}}
             <a href="/dashboard" class="btn btn-ghost btn-sm">Dashboard</a>
             <a href="/pki" class="btn btn-ghost btn-sm">PKI</a>
+            {{if .HasSSHCA}}<a href="/sshca" class="btn btn-ghost btn-sm">SSH CA</a>{{end}}
+            {{if .HasTransit}}<a href="/transit" class="btn btn-ghost btn-sm">Transit</a>{{end}}
+            {{if .HasUser}}<a href="/user" class="btn btn-ghost btn-sm">User Crypto</a>{{end}}
             {{if .IsAdmin}}<a href="/policy" class="btn btn-ghost btn-sm">Policy</a>{{end}}
             <span class="topnav-user">{{.Username}}</span>
             {{if .IsAdmin}}<span class="badge">admin</span>{{end}}