feat: add email notification for post like

migrations/0032_like_email_prefs.down.sql

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+ALTER TABLE users DROP COLUMN IF EXISTS like_emails_enabled;
+ALTER TABLE notifications DROP COLUMN IF EXISTS like_email_sent_at;

migrations/0032_like_email_prefs.up.sql

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+-- Aggregated like emails.
+--
+-- like_email_sent_at throttles the per-post like email: once an aggregated
+-- "X and N others liked your post" email goes out, further likes within the
+-- throttle window only update the in-app/push aggregate, not a fresh email.
+ALTER TABLE notifications ADD COLUMN IF NOT EXISTS like_email_sent_at TIMESTAMPTZ;
+
+-- Per-user opt-out for like emails (push + in-app are unaffected).
+ALTER TABLE users ADD COLUMN IF NOT EXISTS like_emails_enabled BOOLEAN NOT NULL DEFAULT TRUE;