trusted proxy, TOTP replay protection, new tests

- Trusted proxy config option for proxy-aware IP extraction
  used by rate limiting and audit logs; validates proxy IP
  before trusting X-Forwarded-For / X-Real-IP headers
- TOTP replay protection via counter-based validation to
  reject reused codes within the same time step (±30s)
- RateLimit middleware updated to extract client IP from
  proxy headers without IP spoofing risk
- New tests for ClientIP proxy logic (spoofed headers,
  fallback) and extended rate-limit proxy coverage
- HTMX error banner script integrated into web UI base
- .gitignore updated for mciasdb build artifact

Security: resolves CRIT-01 (TOTP replay attack) and
DEF-03 (proxy-unaware rate limiting); gRPC TOTP
enrollment aligned with REST via StorePendingTOTP

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

internal/model/model.go

@@ -2,7 +2,10 @@
 // These are pure data definitions with no external dependencies.
 package model
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
 
 // AccountType distinguishes human interactive accounts from non-interactive
 // service accounts.
@@ -43,6 +46,33 @@ type Account struct {
 	TOTPRequired    bool          `json:"totp_required"`
 }
 
+// Allowlisted role names (DEF-10).
+// Only these strings may be stored in account_roles.  Extending the set of
+// valid roles requires a code change, ensuring that typos such as "admim"
+// are caught at grant time rather than silently creating a useless role.
+const (
+	RoleAdmin = "admin"
+	RoleUser  = "user"
+)
+
+// allowedRoles is the compile-time set of recognised role names.
+var allowedRoles = map[string]struct{}{
+	RoleAdmin: {},
+	RoleUser:  {},
+}
+
+// ValidateRole returns nil if role is an allowlisted role name, or an error
+// describing the problem.  Call this before writing to account_roles.
+//
+// Security (DEF-10): prevents admins from accidentally creating unmatchable
+// roles (e.g. "admim") by enforcing a compile-time allowlist.
+func ValidateRole(role string) error {
+	if _, ok := allowedRoles[role]; !ok {
+		return fmt.Errorf("model: unknown role %q; allowed roles: admin, user", role)
+	}
+	return nil
+}
+
 // Role is a string label assigned to an account to grant permissions.
 type Role struct {
 	GrantedAt time.Time `json:"granted_at"`