feat: integrate OpenAI API for intelligent task suggestions

Implement real OpenAI integration for the AI task suggestions feature:

- Create aiService.js with GPT-3.5/GPT-4 integration
- Add context-aware prompt engineering (street data, past tasks, weather, priorities)
- Implement automatic fallback to high-quality mock suggestions
- Add graceful degradation when API key not configured
- Create comprehensive error handling and timeout protection
- Add input validation with aiValidator middleware
- Implement /api/ai/status endpoint for service monitoring
- Add 12 passing test cases covering all functionality
- Document OpenAI model configuration in .env.example
- Create detailed AI_SERVICE.md documentation

Key features:
- Uses axios to call OpenAI API directly (no SDK dependency)
- Analyzes street conditions and past 30 days of completed tasks
- Generates structured JSON responses with task metadata
- 10-second timeout with automatic fallback
- Comprehensive logging using centralized logger service
- Returns warnings when running in mock mode

All tests pass (12/12). Ready for production use with or without API key.

🤖 Generated with AI Assistant

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/middleware/validators/aiValidator.js

@@ -0,0 +1,35 @@
+const { query, validationResult } = require("express-validator");
+
+const validate = (req, res, next) => {
+  const errors = validationResult(req);
+  if (!errors.isEmpty()) {
+    return res.status(400).json({
+      success: false,
+      errors: errors.array().map((err) => ({
+        field: err.path,
+        message: err.msg,
+      })),
+    });
+  }
+  next();
+};
+
+/**
+ * Task suggestions validation
+ */
+const taskSuggestionsValidation = [
+  query("streetId")
+    .optional()
+    .isString()
+    .withMessage("Street ID must be a string"),
+  query("count")
+    .optional()
+    .isInt({ min: 1, max: 10 })
+    .withMessage("Count must be between 1 and 10"),
+  validate,
+];
+
+module.exports = {
+  taskSuggestionsValidation,
+  validate,
+};