feat(backend): implement comments, image uploads, and data consistency

Implement additional backend features and improve data models:

Comments System:
- Create Comment model with user and post relationships
- Add comments routes: GET /api/posts/:postId/comments (paginated), POST (create), DELETE (own comments)
- Update Post model with commentsCount field
- Emit Socket.IO events for newComment and commentDeleted
- Pagination support for comment lists
- Authorization checks (users can only delete own comments)
- 500 character limit on comments

Image Upload System:
- Implement Cloudinary configuration (config/cloudinary.js)
- Add uploadImage() and deleteImage() helper functions
- Image optimization: max 1000x1000, auto quality, auto format (WebP)
- Integrate image upload in users routes (profile pictures)
- Integrate image upload in posts routes (post images with add/update endpoints)
- File validation: 5MB limit, JPG/PNG/GIF/WebP only
- Automatic image deletion when removing posts/reports

Data Consistency Improvements:
- Add cascade deletes in Street model (remove from user, delete associated tasks)
- Add cascade deletes in Task model (remove from user completedTasks)
- Add cascade deletes in Post model (remove from user posts)
- Update user relationships on save (adoptedStreets, completedTasks, posts, events)
- Add proper indexes for performance (2dsphere for location, compound indexes)
- Add virtual relationships and toJSON configurations

Model Updates:
- Street: Add cascade hooks, location 2dsphere index
- Task: Add cascade hooks, compound indexes for queries
- Post: Add imageUrl, cloudinaryPublicId, commentsCount fields
- Event: Add participants tracking
- Report: Add image upload support
- User: Add earnedBadges virtual, profilePicture, cloudinaryPublicId

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/models/Post.js

@@ -6,6 +6,7 @@ const PostSchema = new mongoose.Schema(
       type: mongoose.Schema.Types.ObjectId,
       ref: "User",
       required: true,
+      index: true,
     },
     content: {
       type: String,
@@ -14,16 +15,48 @@ const PostSchema = new mongoose.Schema(
     imageUrl: {
       type: String,
     },
+    cloudinaryPublicId: {
+      type: String,
+    },
     likes: [
       {
         type: mongoose.Schema.Types.ObjectId,
         ref: "User",
       },
     ],
+    commentsCount: {
+      type: Number,
+      default: 0,
+    },
   },
   {
     timestamps: true,
   },
 );
 
+// Index for querying posts by creation date
+PostSchema.index({ createdAt: -1 });
+
+// Update user relationship when post is created
+PostSchema.post("save", async function (doc) {
+  const User = mongoose.model("User");
+
+  // Add post to user's posts if not already there
+  await User.updateOne(
+    { _id: doc.user },
+    { $addToSet: { posts: doc._id } }
+  );
+});
+
+// Cascade cleanup when a post is deleted
+PostSchema.pre("deleteOne", { document: true, query: false }, async function () {
+  const User = mongoose.model("User");
+
+  // Remove post from user's posts
+  await User.updateOne(
+    { _id: this.user },
+    { $pull: { posts: this._id } }
+  );
+});
+
 module.exports = mongoose.model("Post", PostSchema);