feat(store): default auto_summarize ON for new users (ADR-018)

Migration 011 flips the auto_summarize column default to TRUE and brings
existing rows (maintainer + current registrations) along. Onboarded friends
now get zero-friction discovery: scheduled discovery (ADR-018) both discovers
AND summarizes new videos, so a user's list fills and summarizes itself
instead of presenting an empty list of manual Summarize buttons.

Safe only because the process-wide caption-fetch rate gate (ADR-014 item 2,
prior commit) now exists — auto + scheduled + multi-user would otherwise
self-inflict 429s every cycle. The down migration reverts the default but
intentionally leaves existing rows as-is (no surprise manual regression on
rollback). RegisterUser already lets the column default drive the value, so
no app change is needed; the account-page manual toggle still works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/adapters/store/migrations/011_auto_summarize_default.down.sql

@@ -0,0 +1,5 @@
+-- Revert the column default to FALSE. Existing rows are intentionally NOT
+-- reverted: flipping live users back to manual on a rollback would be a
+-- surprising regression (they may have come to rely on auto). The default change
+-- is the reversible part; data stays as the user left it.
+ALTER TABLE users ALTER COLUMN auto_summarize SET DEFAULT FALSE;