chore: add IANA timezone column to site schema

[anonymoususer72041]

Summary

This PR adds a new time_zone_iana column to the site table.

Fresh installations now create the column as part of the base schema and existing installations receive it through schema update. The column defaults to UTC.

This PR intentionally does not change the existing legacy site.time_zone column, installer behavior, runtime timezone handling, timestamp storage or timestamp display behavior.

Motivation

OpenCATS currently relies on a legacy numeric timezone offset. This is not sufficient for future timezone-aware timestamp handling, especially if timestamp storage is later migrated toward UTC and display logic needs to use real regional timezone rules.

Before the installer or settings screens can start writing an IANA timezone identifier, the database schema must provide a place to store it. This is especially important for upgraded installations: the installer may run before later schema migrations or setup changes can depend on the new field. Without adding the column first, future installer or upgrade logic would not be able to reliably persist the selected IANA timezone for both fresh and existing instances.

This PR is therefore a small preparatory schema change. It makes site.time_zone_iana available while keeping all existing offset-based behavior unchanged. The actual transition to writing IANA timezone values, changing runtime timezone handling, migrating timestamps to UTC, ..., are intentionally left for follow-up PRs.

[anonymoususer72041]

@RussH we want to merge this PR rather sooner than later as it needs to be included in a previous release regarding upcoming UTC work.