"remaining connection slots are reserved": the 7 superuser slots

# "remaining connection slots are reserved": diagnose and fix

## Symptom
`FATAL: remaining connection slots are reserved for roles with the SUPERUSER attribute` (sometimes `for non-replication superuser connections`).

## Cause
Neon reserves 7 connections for its superuser account. When every other slot is taken, regular roles get this instead of "too many clients already". It is the same exhaustion, one step later.

## Confirm
Same as too-many-clients: `SHOW max_connections`, count `pg_stat_activity`, check the Console Monitoring graph. On a 0.25 CU compute the app budget is 104 - 7 = 97 connections.

## Fix
- Move app traffic to the pooled string; this is almost always a direct-connection workload that should not be direct.
- Audit for leaked or long-held direct connections (migrations left open, scripts, BI tools on direct).
- Raise compute size if the direct workload is legitimate.

## Verify
Direct connection count well under max_connections minus 7 at peak.