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

[daria-sukhareva]

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