Add policy-based authz and token delegation

- Replace requireAdmin (role-based) guards on all REST endpoints
  with RequirePolicy middleware backed by the existing policy engine;
  built-in admin wildcard rule (-1) preserves existing admin behaviour
  while operator rules can now grant targeted access to non-admin
  accounts (e.g. a system account allowed to list accounts)
- Wire policy engine into Server: loaded from DB at startup,
  reloaded after every policy-rule create/update/delete so changes
  take effect immediately without a server restart
- Add service_account_delegates table (migration 000008) so a human
  account can be delegated permission to issue tokens for a specific
  system account without holding the admin role
- Add token-download nonce mechanism: a short-lived (5 min),
  single-use random nonce is stored server-side after token issuance;
  the browser downloads the token as a file via
  GET /token/download/{nonce} (Content-Disposition: attachment)
  instead of copying from a flash message
- Add /service-accounts UI page for non-admin delegates
- Add TestPolicyEnforcement and TestPolicyDenyRule integration tests

Security:
- Policy engine uses deny-wins, default-deny semantics; admin wildcard
  is a compiled-in built-in and cannot be deleted via the API
- Token download nonces are 128-bit crypto/rand values, single-use,
  and expire after 5 minutes; a background goroutine evicts stale entries
- alg header validation and Ed25519 signing unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/templates/fragments/token_list.html

@@ -1,5 +1,16 @@
 {{define "token_list"}}
 <div id="token-list">
+  {{if .Flash}}
+  <div class="alert alert-success" role="alert" style="margin-bottom:1rem">
+    {{.Flash}}
+    {{if .DownloadNonce}}
+    <div style="margin-top:.5rem">
+      <a class="btn btn-sm btn-secondary"
+         href="/token/download/{{.DownloadNonce}}">Download token as file</a>
+    </div>
+    {{end}}
+  </div>
+  {{end}}
   {{if .Tokens}}
   <div class="table-wrapper">
     <table>