Add audit logging for all mutating gRPC operations

Log Info-level audit events on success for:
- system: Init, Unseal, Seal
- auth: Login, Logout
- engine: Mount, Unmount
- policy: CreatePolicy, DeletePolicy
- ca: ImportRoot, CreateIssuer, DeleteIssuer, IssueCert, RenewCert

Each log line includes relevant identifiers (mount, issuer, serial, CN,
SANs, username) so that certificate issuance and other privileged
operations are traceable in the server logs.

Co-authored-by: Junie <junie@jetbrains.com>

internal/webserver/client.go

@@ -284,6 +284,8 @@ type CertSummary struct {
 	Issuer     string
 	CommonName string
 	Profile    string
+	IssuedBy   string
+	IssuedAt   string
 	ExpiresAt  string
 }
 
@@ -300,6 +302,10 @@ func (c *VaultClient) ListCerts(ctx context.Context, token, mount string) ([]Cer
 			Issuer:     s.Issuer,
 			CommonName: s.CommonName,
 			Profile:    s.Profile,
+			IssuedBy:   s.IssuedBy,
+		}
+		if s.IssuedAt != nil {
+			cs.IssuedAt = s.IssuedAt.AsTime().Format("2006-01-02T15:04:05Z")
 		}
 		if s.ExpiresAt != nil {
 			cs.ExpiresAt = s.ExpiresAt.AsTime().Format("2006-01-02T15:04:05Z")