Allow scheduling subscriptions at exact times (e.g., 7:05 AM)

**Our problem**
If I’m not mistaken, currently Metabase only supports scheduling dashboard subscriptions at fixed times (e.g., 7:00 AM or 8:00 AM).

**Use case**
We’ve built a dashboard that we need to send to our executives as early as possible. We receive the data at 7:00 AM, and after our pipeline runs the dashboard is ready around 7:02. If we schedule it for 7:00, the dashboard isn’t ready yet. If we choose 8:00 AM we lose almost an hour and executives may not look at it.

**Proposed solution**
We’d like to be able to specify an exact time (e.g., 7:05 AM) when scheduling a subscription.

This new feature would help us a lot!

No file or test is named in the issue. Start by locating the existing dashboard subscription scheduling flow, then trace how scheduled times are represented and selected. Done means a subscription can be configured for an exact time such as 7:05 AM, with coverage for the new scheduling behavior.

Written by the indexing model from the issue text.