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/User.js

@@ -22,6 +22,7 @@ const UserSchema = new mongoose.Schema(
     points: {
       type: Number,
       default: 0,
+      min: 0,
     },
     adoptedStreets: [
       {
@@ -35,11 +36,43 @@ const UserSchema = new mongoose.Schema(
         ref: "Task",
       },
     ],
-    badges: [String],
+    posts: [
+      {
+        type: mongoose.Schema.Types.ObjectId,
+        ref: "Post",
+      },
+    ],
+    events: [
+      {
+        type: mongoose.Schema.Types.ObjectId,
+        ref: "Event",
+      },
+    ],
+    profilePicture: {
+      type: String,
+    },
+    cloudinaryPublicId: {
+      type: String,
+    },
   },
   {
     timestamps: true,
   },
 );
 
+// Indexes for performance
+UserSchema.index({ email: 1 });
+UserSchema.index({ points: -1 }); // For leaderboards
+
+// Virtual for earned badges (populated from UserBadge collection)
+UserSchema.virtual("earnedBadges", {
+  ref: "UserBadge",
+  localField: "_id",
+  foreignField: "user",
+});
+
+// Ensure virtuals are included when converting to JSON
+UserSchema.set("toJSON", { virtuals: true });
+UserSchema.set("toObject", { virtuals: true });
+
 module.exports = mongoose.model("User", UserSchema);