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>

internal/model/model.go

@@ -210,8 +210,25 @@ const (
 	EventPGCredAccessRevoked = "pgcred_access_revoked" //nolint:gosec // G101: audit event type, not a credential
 
 	EventPasswordChanged = "password_changed"
+
+	EventTokenDelegateGranted = "token_delegate_granted"
+	EventTokenDelegateRevoked = "token_delegate_revoked"
 )
 
+// ServiceAccountDelegate records that a specific account has been granted
+// permission to issue tokens for a given system account.  Only admins can
+// add or remove delegates; delegates can issue/rotate tokens for that specific
+// system account and nothing else.
+type ServiceAccountDelegate struct {
+	GrantedAt   time.Time `json:"granted_at"`
+	GrantedBy   *int64    `json:"-"`
+	GranteeUUID string    `json:"grantee_id"`
+	GranteeName string    `json:"grantee_username"`
+	ID          int64     `json:"-"`
+	AccountID   int64     `json:"-"`
+	GranteeID   int64     `json:"-"`
+}
+
 // PolicyRuleRecord is the database representation of a policy rule.
 // RuleJSON holds a JSON-encoded policy.RuleBody (all match and effect fields).
 // The ID, Priority, and Description are stored as dedicated columns.