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/user_key_detail.html

@@ -0,0 +1,30 @@
+{{define "title"}} - User Key: {{.KeyInfo.Username}}{{end}}
+{{define "content"}}
+<div class="page-header">
+    <h2>User Key: {{.KeyInfo.Username}}</h2>
+    <div class="page-meta">
+        <a href="/user">&larr; User Crypto</a>
+    </div>
+</div>
+
+<div class="card">
+    <div class="card-title">Public Key</div>
+    <table class="kv-table">
+        <tbody>
+            <tr><th>Username</th><td>{{.KeyInfo.Username}}</td></tr>
+            <tr><th>Algorithm</th><td><code>{{.KeyInfo.Algorithm}}</code></td></tr>
+            <tr><th>Public Key</th><td><code style="word-break:break-all;">{{.KeyInfo.PublicKey}}</code></td></tr>
+        </tbody>
+    </table>
+</div>
+
+{{if .IsAdmin}}
+<div class="card">
+    <div class="card-title">Admin Actions</div>
+    <form method="post" action="/user/delete/{{.KeyInfo.Username}}">
+        {{csrfField}}
+        <button type="submit" class="btn-danger" onclick="return confirm('Delete user {{.KeyInfo.Username}} and their keys? This cannot be undone.')">Delete User</button>
+    </form>
+</div>
+{{end}}
+{{end}}