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/PointTransaction.js

@@ -0,0 +1,57 @@
+const mongoose = require("mongoose");
+
+const PointTransactionSchema = new mongoose.Schema(
+  {
+    user: {
+      type: mongoose.Schema.Types.ObjectId,
+      ref: "User",
+      required: true,
+      index: true,
+    },
+    amount: {
+      type: Number,
+      required: true,
+    },
+    type: {
+      type: String,
+      enum: [
+        "street_adoption",
+        "task_completion",
+        "post_creation",
+        "event_participation",
+        "reward_redemption",
+        "admin_adjustment",
+      ],
+      required: true,
+      index: true,
+    },
+    description: {
+      type: String,
+      required: true,
+    },
+    relatedEntity: {
+      entityType: {
+        type: String,
+        enum: ["Street", "Task", "Post", "Event", "Reward"],
+      },
+      entityId: {
+        type: mongoose.Schema.Types.ObjectId,
+      },
+    },
+    balanceAfter: {
+      type: Number,
+      required: true,
+    },
+  },
+  {
+    timestamps: true,
+  },
+);
+
+// Compound index for user transaction history queries
+PointTransactionSchema.index({ user: 1, createdAt: -1 });
+
+// Index for querying by transaction type
+PointTransactionSchema.index({ type: 1, createdAt: -1 });
+
+module.exports = mongoose.model("PointTransaction", PointTransactionSchema);