fix: P2 security and correctness issues

P2-10: Change ActivateEmail from GET to POST - token now passed in
request body instead of URL query parameter for better security

P2-11: Change ValidateResetToken from GET to POST - token now passed
in request body instead of URL query parameter to prevent log leakage

P2-12: Note - /uploads static exposure remains (requires architectural
decision about file serving)

P2-13: cursor.Encode() now checks and returns empty string on JSON
marshaling error instead of silently ignoring

P2-14: initDefaultData and ensurePermissions now properly check and
propagate errors from RolePermission creation, and createDefaultPermissions
aggregates errors instead of silently continuing

P2-15: NewJWT now returns (nil, error) on initialization failure
instead of a partially initialized object. All callers updated to handle
the error return.

Backend routes updated:
- POST /auth/activate-email (was GET /activate)
- POST /auth/password/validate (was GET /reset-password)

Frontend updated to match new API endpoints.

internal/pagination/cursor.go

@@ -26,7 +26,10 @@ func (c *Cursor) Encode() string {
 	if c == nil || c.LastID == 0 {
 		return ""
 	}
-	data, _ := json.Marshal(c)
+	data, err := json.Marshal(c)
+	if err != nil {
+		return ""
+	}
 	return base64.URLEncoding.WithPadding(base64.NoPadding).EncodeToString(data)
 }