Postgresql/TimescaleDB datasource - Min time interval do not take into account

### What happened?

Hi all,

i have a TimescaleDB data source configured with the following options:

When i try to use the $__interval variable in an SQL query:

```
SELECT
$__timeGroupAlias(ref_timestamp, $__interval),
AVG(value)
FROM database
WHERE $__timeFilter(ref_timestamp) AND tag = 'EXAMPLE'
GROUP BY time
```

it is translated to the following request in the Grafana Query Inspector:

```
SELECT
time_bucket('60.000s',ref_timestamp) AS "time",
AVG(value)
FROM database
WHERE ref_timestamp BETWEEN '2026-02-08T10:22:15.122Z' AND '2026-02-09T10:22:15.122Z' AND tag = 'EXAMPLE'
GROUP BY time
```
$__interval always return a value equal or higher than '60.000s' regardless to the selected time period.

I also try to configure a custom interval variable as following:

But only value higher than 60s are displayed in the menu:

PS: my dashboard did not have any query option configured:

Thanks !

### What did you expect to happen?

I expect to have a minimum value of '5s' (for $__interval variable or for my custom variable) when i zoom on my data.

### Did this work before?

First time i try this.

### How do we reproduce it?

1. Create a TimescaleDB datasource with Min interval set to 5s (five seconds)
2. Create a dashboard with a $__interval time query
3. Zoom...
4. Sampling period is never lower than 60s

### Is the bug inside a dashboard panel?

Not related to the panel

### Environment (with versions)?

Grafana: Grafana v12.3.1
OS: Docker image
Browser: Chrome

### Grafana platform?

Docker

### Datasource(s)?

TimescaleDB

Reproduce the TimescaleDB datasource query with a 5s minimum interval and inspect the generated SQL in Query Inspector while zooming the dashboard. Trace the datasource's interval handling from the $__interval and custom interval inputs; done means values below 60s, including 5s, are honored when the selected range requires them.

Written by the indexing model from the issue text.