feat(backend): implement complete gamification system

Implement comprehensive points and badges system with MongoDB transactions:

Point System:
- Create PointTransaction model for transaction history
- Award points atomically using MongoDB transactions
- Point values: street adoption (+100), task completion (+50), post creation (+10), event participation (+75)
- Track balance after each transaction
- Support point deduction for reward redemption

Badge System:
- Create Badge and UserBadge models
- Define badge criteria types: street_adoptions, task_completions, post_creations, event_participations, points_earned
- Auto-award badges based on user achievements
- Badge rarity levels: common, rare, epic, legendary
- Track badge progress for users
- Prevent duplicate badge awards

Gamification Service:
- Implement gamificationService.js with 390 lines of logic
- awardPoints() with transaction support
- checkAndAwardBadges() for auto-awarding
- getUserBadgeProgress() for progress tracking
- getUserStats() for achievement statistics
- Atomic operations prevent double-awarding

Integration:
- Streets route: Award points and badges on adoption
- Tasks route: Award points and badges on completion
- Posts route: Award points and badges on creation
- Events route: Award points and badges on RSVP
- Rewards route: Deduct points on redemption
- Badges API: List badges, track progress, view earned badges

Updated User Model:
- Add points field (default 0)
- Add earnedBadges virtual relationship
- Add indexes for performance (points for leaderboards)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/routes/tasks.js

@@ -1,25 +1,53 @@
 const express = require("express");
+const mongoose = require("mongoose");
 const Task = require("../models/Task");
+const User = require("../models/User");
 const auth = require("../middleware/auth");
+const { asyncHandler } = require("../middleware/errorHandler");
+const {
+  createTaskValidation,
+  taskIdValidation,
+} = require("../middleware/validators/taskValidator");
+const {
+  awardTaskCompletionPoints,
+  checkAndAwardBadges,
+} = require("../services/gamificationService");
 
 const router = express.Router();
 
-// Get all tasks for user
-router.get("/", auth, async (req, res) => {
-  try {
-    const tasks = await Task.find({ completedBy: req.user.id });
-    res.json(tasks);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+// Get all tasks for user (with pagination)
+router.get(
+  "/",
+  auth,
+  asyncHandler(async (req, res) => {
+    const { paginate, buildPaginatedResponse } = require("../middleware/pagination");
+
+    // Parse pagination params
+    const page = parseInt(req.query.page) || 1;
+    const limit = Math.min(parseInt(req.query.limit) || 10, 100);
+    const skip = (page - 1) * limit;
+
+    const tasks = await Task.find({ completedBy: req.user.id })
+      .sort({ createdAt: -1 })
+      .skip(skip)
+      .limit(limit)
+      .populate("street", ["name"])
+      .populate("completedBy", ["name"]);
+
+    const totalCount = await Task.countDocuments({ completedBy: req.user.id });
+
+    res.json(buildPaginatedResponse(tasks, totalCount, page, limit));
+  }),
+);
 
 // Create a task
-router.post("/", auth, async (req, res) => {
-  const { street, description } = req.body;
+router.post(
+  "/",
+  auth,
+  createTaskValidation,
+  asyncHandler(async (req, res) => {
+    const { street, description } = req.body;
 
-  try {
     const newTask = new Task({
       street,
       description,
@@ -27,30 +55,70 @@ router.post("/", auth, async (req, res) => {
 
     const task = await newTask.save();
     res.json(task);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+  }),
+);
 
 // Complete a task
-router.put("/:id", auth, async (req, res) => {
-  try {
-    const task = await Task.findById(req.params.id);
-    if (!task) {
-      return res.status(404).json({ msg: "Task not found" });
+router.put(
+  "/:id",
+  auth,
+  taskIdValidation,
+  asyncHandler(async (req, res) => {
+    const session = await mongoose.startSession();
+    session.startTransaction();
+
+    try {
+      const task = await Task.findById(req.params.id).session(session);
+      if (!task) {
+        await session.abortTransaction();
+        session.endSession();
+        return res.status(404).json({ msg: "Task not found" });
+      }
+
+      // Check if task is already completed
+      if (task.status === "completed") {
+        await session.abortTransaction();
+        session.endSession();
+        return res.status(400).json({ msg: "Task already completed" });
+      }
+
+      // Update task
+      task.completedBy = req.user.id;
+      task.status = "completed";
+      await task.save({ session });
+
+      // Update user's completedTasks array
+      const user = await User.findById(req.user.id).session(session);
+      if (!user.completedTasks.includes(task._id)) {
+        user.completedTasks.push(task._id);
+        await user.save({ session });
+      }
+
+      // Award points for task completion
+      const { transaction } = await awardTaskCompletionPoints(
+        req.user.id,
+        task._id,
+        session,
+      );
+
+      // Check and award badges
+      const newBadges = await checkAndAwardBadges(req.user.id, session);
+
+      await session.commitTransaction();
+      session.endSession();
+
+      res.json({
+        task,
+        pointsAwarded: transaction.amount,
+        newBalance: transaction.balanceAfter,
+        badgesEarned: newBadges,
+      });
+    } catch (err) {
+      await session.abortTransaction();
+      session.endSession();
+      throw err;
     }
-
-    task.completedBy = req.user.id;
-    task.status = "completed";
-
-    await task.save();
-
-    res.json(task);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+  }),
+);
 
 module.exports = router;