Rate limits

Implement per-project rate limiter

## Limitting

Max N events per T for Project 

N and T should be configurable. The best way is to include them to the Plans so each plan can have own values

Initially set these values:

N — 5000 events
T — 5 min

After T time passed, project should continue accept events.

## Statistic

Collector should increment rate limited events counter of the project.

API should be able to get that data **grouped by hour**. We will use it to display them on the Project Events graph like this:

### Questions:

1. Where to store statistic? Mongo or Redis? Both Collector and Api should be able to access it
2. How to store statistic? For example, we can store timestamp without minutes in Mongo — 1 record for 1 hour (`workspace_id | project_id | timestamp-hourly | counter`). Or key like `rate-limited:workspaceId::projectId::`. Or something other.
3. Who should insert statistic? Collector itself or separate worker?

Start by examining the Collector and API paths that handle project events and the Plans configuration, then resolve the open storage and ownership questions for rate-limit statistics. Done means configurable per-project limits, the initial 5000-events-per-5-minutes values, continued acceptance after the interval, and hourly grouped rate-limit data exposed by the API.

Written by the indexing model from the issue text.