Performance: CSV import loads entire file into memory with no size limit

## Description

In `app/models/subscribers_import.rb:14-25`, `CSV.parse(f.read, headers: false)` loads the entire CSV file into memory with no file size validation.

## Risk

A large CSV upload could exhaust server memory (DoS).

## Recommendation

- Validate file size before processing (e.g., max 5MB)
- Use streaming CSV parsing (`CSV.foreach`) instead of loading all at once

## Severity

Medium