[Audit] Declare grain explicitly in model descriptions for dim_ports, fact_charge_attempts, fact_downtime_daily, fact_interval_data

## Context

Four models have descriptions that do not state their grain. Grain is the single most important piece of documentation on any dimensional model — it tells consumers exactly what one row represents, prevents incorrect aggregations, and is the first thing a reviewer checks when a metric looks wrong.

The current state:

| Model | Current description | Required grain statement |
|---|---|---|
| `dim_ports` | "Port/connector dimension" | "One row per charge_point_id + port_id + connector_id" |
| `fact_charge_attempts` | "combines charge attempts and transactions data" | "One row per charge attempt per port" |
| `fact_downtime_daily` | "Daily aggregation of charge point downtime" | "One row per date + charge_point_id + port_id + outage type" |
| `fact_interval_data` | "15-minute interval meter values" | "One row per charge_point_id + transaction_id + connector_id + ingested_ts + measurand + unit + phase + 15-min interval" |

`fact_uptime` is the only model that currently states its grain correctly and does not need to change.

## Acceptance criteria

- [ ] `dim_ports` model description begins with a grain statement in the form "One row per …"
- [ ] `fact_charge_attempts` model description begins with a grain statement in the form "One row per …"
- [ ] `fact_downtime_daily` model description begins with a grain statement in the form "One row per …"
- [ ] `fact_interval_data` model description begins with a grain statement in the form "One row per …"
- [ ] Each grain statement is specific enough that two engineers would independently produce the same unique key definition from it
- [ ] Grain statements are consistent with the unique key columns actually tested in each model's `.yml` config
- [ ] No SQL changes are required for this issue — documentation only