DC-059: Joi validation middleware + schemas for destructive routes

[grade=B]

- New src/utilities/validate.js: validateBody(schema) middleware + 9 schemas
  (backupConfigUpdate, backupScheduleCreate, backupRestore, backupRestoreFile,
   appDeploy, appRestore, appRevert, assetUpload, logoUpload)
- Uses Joi's authoritative CIDR validator (rejects malformed IPv6 like ::::/64
  that the previous hex/colon regex would have accepted)
- appDeploy.config uses .unknown(true) for forward-compat with template-specific
  fields (sslType, dnsType, plexClaimToken, etc.) — preserves fields the live
  frontend posts, prevents a behavioural regression
- appRestore uses Joi.any().custom() so the empty-body semantics hold under
  middleware stripUnknown (default) — body with extra keys now rejected
- Wired into 8 destructive routes: backups schedule/restore/config, apps
  deploy/restore/revert, assets upload/logo
- Duplicate legacy POST /backups/schedule handler (line 519) marked LEGACY
  with TODO removal note (Express only matches first registration; this
  handler is unreachable under normal routing)
- Removed redundant manual appId check in /backups/schedule (Joi schema
  enforces it)
- Removed unused 'mime' destructure in /assets/favicon (decodeImageData
  validates MIME internally)
- 41 unit tests covering every exported schema + middleware integration
- 1539/1539 Jest tests pass, zero new ESLint warnings

dashcaddy-api/routes/apps/deploy.js

@@ -9,6 +9,7 @@ const platformPaths = require('../../platform-paths');
 const { ValidationError } = require('../../src/utilities/errors');
 const { logError } = require('../../src/utils/logging');
 const { ok } = require('../../src/utils/responses');
+const { validateBody, schemas: valSchemas } = require('../../src/utilities/validate');
 /**
  * Apps deployment routes factory
  * @param {Object} deps - Explicit dependencies
@@ -251,17 +252,8 @@ module.exports = function({ docker, caddy, credentialManager, servicesStateManag
   }, 'check-existing'));
 
   // Deploy new app
-  router.post('/deploy', asyncHandler(async (req, res) => {
+  router.post('/deploy', validateBody(valSchemas.appDeploy), asyncHandler(async (req, res) => {
     const { appId, config } = req.body;
-    if (!appId || typeof appId !== 'string') {
-      throw new ValidationError('appId is required');
-    }
-    if (!config || typeof config !== 'object') {
-      throw new ValidationError('config object is required');
-    }
-    if (!config.subdomain || typeof config.subdomain !== 'string') {
-      throw new ValidationError('config.subdomain is required');
-    }
     try {
       log.info('deploy', 'Deploying app', { appId, subdomain: config.subdomain });
       const template = ctx.APP_TEMPLATES[appId];