feat: add strong_migrations gem to prevent unsafe migration deploys

[mroderick]

What

Adds the strong_migrations gem to block dangerous migration patterns before they reach production.

Scope

• Checks all migrations from 2026-01-01 onwards (start_after = 20260101000000)
• Targets PostgreSQL 16 (Heroku production version)
• Active in all environments including test (catches issues in CI)

Existing violations acknowledged

Six 2026 migrations already deployed contain patterns strong_migrations flags. Each is wrapped in safety_assured { ... } — no logic changes, just marking them as reviewed:

• execute with DELETE for duplicate cleanup
• update_all inside a schema migration for data backfill
• add_index without algorithm: :concurrently
• remove_index without algorithm: :concurrently
• drop_table on meeting_talks
• Data cleanup loops (delete_all) inside schema migrations

Reviewer hint: it's a lot easier to review this part of the PR, if you hide the whitespace changes 😄

What this prevents going forward

• Adding non-nullable columns without defaults to existing tables
• Adding indexes without algorithm: :concurrently on large tables
• Data backfills inside schema migrations
• drop_table / remove_column without a staged removal process
• Raw execute SQL without review

[mroderick]

@olleolleolle do you think it makes sense to fix all the existing migrations, going all the way back to the dawn of time?