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/models/Badge.js

@@ -0,0 +1,57 @@
+const mongoose = require("mongoose");
+
+const BadgeSchema = new mongoose.Schema(
+  {
+    name: {
+      type: String,
+      required: true,
+      unique: true,
+    },
+    description: {
+      type: String,
+      required: true,
+    },
+    icon: {
+      type: String,
+      required: true,
+    },
+    criteria: {
+      type: {
+        type: String,
+        enum: [
+          "street_adoptions",
+          "task_completions",
+          "post_creations",
+          "event_participations",
+          "points_earned",
+          "consecutive_days",
+          "special",
+        ],
+        required: true,
+      },
+      threshold: {
+        type: Number,
+        required: true,
+      },
+    },
+    rarity: {
+      type: String,
+      enum: ["common", "rare", "epic", "legendary"],
+      default: "common",
+    },
+    order: {
+      type: Number,
+      default: 0,
+    },
+  },
+  {
+    timestamps: true,
+  },
+);
+
+// Index for efficient badge queries
+BadgeSchema.index({ "criteria.type": 1, "criteria.threshold": 1 });
+BadgeSchema.index({ rarity: 1 });
+BadgeSchema.index({ order: 1 });
+
+module.exports = mongoose.model("Badge", BadgeSchema);