Paid plan accounts with many agents were hitting the flat daily email
rate limit cap.
This multiplies the plan base limit by the account's agent seat count,
so larger teams
get proportionally higher limits. Free/hacker plan keeps the flat limit
unchanged.
Fixes https://linear.app/chatwoot/issue/CW-6664
## How to test
1. Set up `ACCOUNT_EMAILS_PLAN_LIMITS` config with plan limits (e.g.,
`{"hacker": 10, "startups": 20, "business": 30, "enterprise": 40}`)
2. Create an account on a paid plan (e.g., startups) with multiple agent
seats
3. Verify `account.email_rate_limit` returns `base_limit × agent_seats`
(e.g., 20 × 5 = 100)
4. Create an account on the hacker plan — verify limit stays flat (10)
5. Set a per-account override via super admin `limits.emails` — verify
it takes priority over the multiplied limit
## What changed
- `plan_email_limit` now multiplies the base plan limit by agent seat
count for paid plans
- Added `free_plan?` helper to skip the multiplier for the default/free
plan
