[grade=B] DC-081: Input validation for 20 highest-risk mutating routes

Secures 20 mutating routes across 7 files against path traversal, shell
injection, and ReDoS vectors:
- containers.js: container ID validation + resource limit bounds (6 routes)
- recipes/manage.js: recipe ID slug validation (4 routes)
- tailscale.js: subdomain regex before interpolation + shell char blocking (2)
- workflows.js: workflow ID slug validation (3 routes)
- dependencies.js: service ID + dependsOn array validation (3 routes)
- logs.js: YYYY-MM-DD date format validation (1 route)
- sites.js: additional domain validation (1 route)

Uses existing REGEX patterns from constants.js. No new dependencies.
Codex: B (no blocking issues, 4 Low follow-ups for tests + strict bools).
1552/1552 tests pass, 0 regressions.

dashcaddy-api/routes/workflows.js

@@ -1,5 +1,20 @@
 const express = require('express');
 const { ok } = require('../src/utils/responses');
+const { ValidationError } = require('../src/utilities/errors');
+
+/**
+ * Validate a workflow ID.
+ * @param {string} workflowId - Workflow ID from route param
+ * @throws {ValidationError} if the ID contains unsafe characters
+ */
+function validateWorkflowId(workflowId) {
+  if (!workflowId || typeof workflowId !== 'string') {
+    throw new ValidationError('Workflow ID is required');
+  }
+  if (!/^[a-zA-Z0-9][a-zA-Z0-9_-]{0,63}$/.test(workflowId)) {
+    throw new ValidationError('Invalid workflow ID format');
+  }
+}
 
 /**
  * Workflows routes factory
@@ -27,6 +42,7 @@ module.exports = function({ workflowEngine, licenseManager, asyncHandler, ok })
   // Enable a workflow
   router.post('/workflows/:workflowId/enable', asyncHandler(async (req, res) => {
     const { workflowId } = req.params;
+    validateWorkflowId(workflowId);
     const result = workflowEngine.setWorkflowEnabled(workflowId, true);
     ok(res, result);
   }, 'workflows-enable'));
@@ -34,6 +50,7 @@ module.exports = function({ workflowEngine, licenseManager, asyncHandler, ok })
   // Disable a workflow
   router.post('/workflows/:workflowId/disable', asyncHandler(async (req, res) => {
     const { workflowId } = req.params;
+    validateWorkflowId(workflowId);
     const result = workflowEngine.setWorkflowEnabled(workflowId, false);
     ok(res, result);
   }, 'workflows-disable'));
@@ -41,6 +58,7 @@ module.exports = function({ workflowEngine, licenseManager, asyncHandler, ok })
   // Manually trigger a workflow
   router.post('/workflows/:workflowId/run', asyncHandler(async (req, res) => {
     const { workflowId } = req.params;
+    validateWorkflowId(workflowId);
     const triggerData = req.body || {};
     triggerData.trigger = 'manual';