Avoid backfilling remote quote policies

[dahlia]

Summary

This PR updates the unreleased quote-control migrations so existing remote posts are not mass-updated when introducing nullable quote approval policies.

The previous migration sequence first added quote_approval_policy as DEFAULT 'public' NOT NULL, then used the follow-up migration to drop NOT NULL and rewrite cached remote public rows to NULL. That worked functionally, but it was expensive on large posts tables and could take many minutes in production-like databases.

This PR makes the migration create the final column shape directly: nullable from the start, with only local posts backfilled. Existing remote posts remain NULL, which preserves the legacy remote-post behavior added by the previous PR without requiring a table-wide remote-row rewrite.

Changes

• Update drizzle/0086_quote_controls.sql to add quote_approval_policy as nullable and without an immediate default.
• Backfill quote_approval_policy only for local posts, where Hollo owns the quote policy and can derive it from visibility.
• Set the default to 'public' only after the local backfill, so new local posts keep the expected default.
• Update drizzle/0087_quote_authorization_requirement.sql to keep the cheap DROP NOT NULL for databases that already applied the earlier 0086 draft, while removing the expensive remote-row backfill.
• Add migration comments documenting why remote posts are intentionally left NULL.

Why this is needed

On instances with many cached remote posts, the old follow-up migration spent too long rewriting remote rows from 'public' to NULL. That rewrite is unnecessary if the column is nullable from the beginning.

It also avoids encoding misleading data: before this feature, cached remote posts did not have stored FEP-044f quote policy information, so defaulting every old remote row to 'public' made legacy remote posts indistinguishable from remote posts that truly advertised a public FEP-044f quote policy.

By leaving existing remote rows NULL, the database accurately represents “no quote policy was observed,” and the application can treat those posts as legacy quote targets.

Testing

• pnpm migrate:generate
• pnpm run migrate:test
• pnpm check
• git diff --check

[dahlia]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

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