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/posts.js

@@ -1,42 +1,154 @@
 const express = require("express");
+const mongoose = require("mongoose");
 const Post = require("../models/Post");
 const auth = require("../middleware/auth");
+const { asyncHandler } = require("../middleware/errorHandler");
+const {
+  createPostValidation,
+  postIdValidation,
+} = require("../middleware/validators/postValidator");
+const { upload, handleUploadError } = require("../middleware/upload");
+const { uploadImage, deleteImage } = require("../config/cloudinary");
+const { paginate, buildPaginatedResponse } = require("../middleware/pagination");
+const {
+  awardPostCreationPoints,
+  checkAndAwardBadges,
+} = require("../services/gamificationService");
 
 const router = express.Router();
 
-// Get all posts
-router.get("/", async (req, res) => {
-  try {
-    const posts = await Post.find().populate("user", ["name"]);
-    res.json(posts);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+// Get all posts (with pagination)
+router.get(
+  "/",
+  paginate,
+  asyncHandler(async (req, res) => {
+    const { skip, limit, page } = req.pagination;
 
-// Create a post
-router.post("/", auth, async (req, res) => {
-  const { content, imageUrl } = req.body;
+    const posts = await Post.find()
+      .sort({ createdAt: -1 })
+      .skip(skip)
+      .limit(limit)
+      .populate("user", ["name", "profilePicture"]);
 
-  try {
-    const newPost = new Post({
-      user: req.user.id,
-      content,
-      imageUrl,
-    });
+    const totalCount = await Post.countDocuments();
+
+    res.json(buildPaginatedResponse(posts, totalCount, page, limit));
+  }),
+);
+
+// Create a post with optional image
+router.post(
+  "/",
+  auth,
+  upload.single("image"),
+  handleUploadError,
+  asyncHandler(async (req, res) => {
+    const { content } = req.body;
+    const session = await mongoose.startSession();
+    session.startTransaction();
+
+    try {
+      if (!content) {
+        await session.abortTransaction();
+        session.endSession();
+        return res.status(400).json({ msg: "Content is required" });
+      }
+
+      const postData = {
+        user: req.user.id,
+        content,
+      };
+
+      // Upload image if provided
+      if (req.file) {
+        const result = await uploadImage(
+          req.file.buffer,
+          "adopt-a-street/posts",
+        );
+        postData.imageUrl = result.url;
+        postData.cloudinaryPublicId = result.publicId;
+      }
+
+      const newPost = new Post(postData);
+      const post = await newPost.save({ session });
+
+      // Award points for post creation
+      const { transaction } = await awardPostCreationPoints(
+        req.user.id,
+        post._id,
+        session
+      );
+
+      // Check and award badges
+      const newBadges = await checkAndAwardBadges(req.user.id, session);
+
+      await session.commitTransaction();
+      session.endSession();
+
+      // Populate user data before sending response
+      await post.populate("user", ["name", "profilePicture"]);
+
+      res.json({
+        post,
+        pointsAwarded: transaction.amount,
+        newBalance: transaction.balanceAfter,
+        badgesEarned: newBadges,
+      });
+    } catch (err) {
+      await session.abortTransaction();
+      session.endSession();
+      throw err;
+    }
+  }),
+);
+
+// Add image to existing post
+router.post(
+  "/:id/image",
+  auth,
+  upload.single("image"),
+  handleUploadError,
+  postIdValidation,
+  asyncHandler(async (req, res) => {
+    const post = await Post.findById(req.params.id);
+    if (!post) {
+      return res.status(404).json({ msg: "Post not found" });
+    }
+
+    // Verify user owns the post
+    if (post.user.toString() !== req.user.id) {
+      return res.status(403).json({ msg: "Not authorized" });
+    }
+
+    if (!req.file) {
+      return res.status(400).json({ msg: "No image file provided" });
+    }
+
+    // Delete old image if exists
+    if (post.cloudinaryPublicId) {
+      await deleteImage(post.cloudinaryPublicId);
+    }
+
+    // Upload new image
+    const result = await uploadImage(
+      req.file.buffer,
+      "adopt-a-street/posts",
+    );
+
+    post.imageUrl = result.url;
+    post.cloudinaryPublicId = result.publicId;
+    await post.save();
 
-    const post = await newPost.save();
     res.json(post);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+  }),
+);
 
 // Like a post
-router.put("/like/:id", auth, async (req, res) => {
-  try {
+router.put(
+  "/like/:id",
+  auth,
+  postIdValidation,
+  asyncHandler(async (req, res) => {
     const post = await Post.findById(req.params.id);
     if (!post) {
       return res.status(404).json({ msg: "Post not found" });
@@ -54,10 +166,7 @@ router.put("/like/:id", auth, async (req, res) => {
     await post.save();
 
     res.json(post.likes);
-  } catch (err) {
-    console.error(err.message);
-    res.status(500).send("Server error");
-  }
-});
+  }),
+);
 
 module.exports = router;