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/badges.js

@@ -0,0 +1,76 @@
+const express = require("express");
+const Badge = require("../models/Badge");
+const UserBadge = require("../models/UserBadge");
+const auth = require("../middleware/auth");
+const { asyncHandler } = require("../middleware/errorHandler");
+const { getUserBadgeProgress } = require("../services/gamificationService");
+
+const router = express.Router();
+
+/**
+ * GET /api/badges
+ * Get all available badges
+ */
+router.get(
+  "/",
+  asyncHandler(async (req, res) => {
+    const badges = await Badge.find().sort({ order: 1, rarity: 1 });
+    res.json(badges);
+  })
+);
+
+/**
+ * GET /api/badges/progress
+ * Get current user's badge progress (requires authentication)
+ */
+router.get(
+  "/progress",
+  auth,
+  asyncHandler(async (req, res) => {
+    const progress = await getUserBadgeProgress(req.user.id);
+    res.json(progress);
+  })
+);
+
+/**
+ * GET /api/users/:userId/badges
+ * Get badges earned by a specific user
+ */
+router.get(
+  "/users/:userId",
+  asyncHandler(async (req, res) => {
+    const { userId } = req.params;
+
+    const userBadges = await UserBadge.find({ user: userId })
+      .populate("badge")
+      .sort({ earnedAt: -1 });
+
+    res.json(
+      userBadges.map((ub) => ({
+        badge: ub.badge,
+        earnedAt: ub.earnedAt,
+        progress: ub.progress,
+      }))
+    );
+  })
+);
+
+/**
+ * GET /api/badges/:badgeId
+ * Get a specific badge by ID
+ */
+router.get(
+  "/:badgeId",
+  asyncHandler(async (req, res) => {
+    const { badgeId } = req.params;
+
+    const badge = await Badge.findById(badgeId);
+    if (!badge) {
+      return res.status(404).json({ msg: "Badge not found" });
+    }
+
+    res.json(badge);
+  })
+);
+
+module.exports = router;