[feature] Ability to set "Time Between Check" per tag/group

**Describe the solution you'd like**
Allow setting the "Time Between Check" interval for each tag or group, so all watches under that tag inherit the same schedule unless overridden individually.

**Describe the use-case and give concrete real-world examples**
Example use cases:

- Rarely updated pages (e.g., informational/policy pages) → check weekly
- Frequently updated pages (e.g., job boards/news) → check every few hours

This would simplify management of large watch lists, reduce manual edits, and help control webhook/notification load. Updating the tag setting would update the schedule for all linked watches.

The issue names no files, tests, or entry points. Start by tracing how tags or groups and per-watch check intervals are stored and applied, then identify the existing schedule-update tests. Done means a tag or group interval is inherited by its watches, individual overrides still work, and changing the tag setting updates linked schedules.

Written by the indexing model from the issue text.