Fix F-08, F-12, F-13: Implement account lockout, username validation, and password minimum length enforcement

- Added failed login tracking for account lockout enforcement in `db` and `ui` layers; introduced `failed_logins` table to store attempts, window start, and attempt count.
- Updated login checks in `grpcserver/auth.go` and `ui/handlers_auth.go` to reject requests if the account is locked.
- Added immediate failure counter reset on successful login.
- Implemented username length and character set validation (F-12) and minimum password length enforcement (F-13) in shared `validate` package.
- Updated account creation and edit flows in `ui` and `grpcserver` layers to apply validation before hashing/processing.
- Added comprehensive unit tests for lockout, validation, and related edge cases.
- Updated `AUDIT.md` to mark F-08, F-12, and F-13 as fixed.
- Updated `openapi.yaml` to reflect new validation and lockout behaviors.

Security: Prevents brute-force attacks via lockout mechanism and strengthens defenses against weak and invalid input.

internal/ui/handlers_accounts.go

@@ -7,6 +7,7 @@ import (
 
 	"git.wntrmute.dev/kyle/mcias/internal/auth"
 	"git.wntrmute.dev/kyle/mcias/internal/model"
+	"git.wntrmute.dev/kyle/mcias/internal/validate"
 )
 
 // knownRoles lists the built-in roles shown as checkboxes in the roles editor.
@@ -44,8 +45,9 @@ func (u *UIServer) handleCreateAccount(w http.ResponseWriter, r *http.Request) {
 	password := r.FormValue("password")
 	accountTypeStr := r.FormValue("account_type")
 
-	if username == "" {
-		u.renderError(w, r, http.StatusBadRequest, "username is required")
+	// Security (F-12): validate username length and character set.
+	if err := validate.Username(username); err != nil {
+		u.renderError(w, r, http.StatusBadRequest, err.Error())
 		return
 	}
 
@@ -56,6 +58,11 @@ func (u *UIServer) handleCreateAccount(w http.ResponseWriter, r *http.Request) {
 
 	var passwordHash string
 	if password != "" {
+		// Security (F-13): enforce minimum length before hashing.
+		if err := validate.Password(password); err != nil {
+			u.renderError(w, r, http.StatusBadRequest, err.Error())
+			return
+		}
 		argonCfg := auth.ArgonParams{
 			Time:    u.cfg.Argon2.Time,
 			Memory:  u.cfg.Argon2.Memory,