Normalize initial database state for new installations

[anonymoususer72041]

Summary

This PR normalizes the initial database state created from db/cats_schema.sql by removing demo/sample tag data, resetting instance-specific seeded values to deterministic defaults, and replacing testdomain.com placeholders with the IANA-reserved placeholder domain example.com.

In addition, it updates modules/install/Schema.php to migrate existing databases where site.last_viewed_day is NULL or 0000-00-00 by backfilling a sentinel date (1000-01-01) and then enforcing a non-null default at the schema level.

Motivation

Fresh installs should start from a clean, deterministic baseline without embedded demo content or leftover environment-specific values. This reduces confusion, makes the initial schema easier to reason about, and avoids noisy diffs when comparing or exporting databases.

Backfilling and normalizing site.last_viewed_day prevents NULL/zero-date edge cases and aligns this field with the sentinel-date approach used elsewhere in the schema while keeping existing instances consistent through an explicit migration step.

Replacing testdomain.com with the IANA-reserved placeholder domain example.com avoids potential confusion with real domains.