feat(podcasts): widen reader_settings key ceiling — item 2.1 foundation

reader_settings.setting_key was VARCHAR(50), and the composite
podcast_order:<uuid> key already sits at exactly 50 chars — zero
headroom. The deferred queue-editor idea (plan §0 item 2b) and any
future per-feed/per-collection preference key would have been forced
to payload-stuff inside the order row by a column width. Composite
prefix:<uuid> keys are the norm now (§10 R5), so the ceiling widens
to VARCHAR(120): idempotent in-place ALTER (varchar widening is
metadata-only, no table rewrite), re-runs harmlessly on every boot
per the schema convention.

No new episode-membership tables: collection_items already carries
ordered playlist membership (position, A2c) against media_items,
which podcast episodes ARE — episodes join collections through the
existing table.

Pinned by TestReaderSettingsCompositeKeyHeadroom: a 57-char composite
key (the podcast_custom_order:<uuid> shape) upserts and reads back.
Under the old width this fails; sqlc codegen is byte-identical.

database/schema/schema.sql

@@ -1628,12 +1628,19 @@ CREATE INDEX IF NOT EXISTS idx_dictionary_word ON dictionary_cache(word);
 CREATE TABLE IF NOT EXISTS reader_settings (
   id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
   user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
-  setting_key VARCHAR(50) NOT NULL,
+  setting_key VARCHAR(120) NOT NULL,
   setting_value JSONB NOT NULL,
   updated_at TIMESTAMPTZ DEFAULT NOW(),
   UNIQUE(user_id, setting_key)
 );
 
+-- Key-width headroom (podcast plan §10 R5, item 2.1): the composite
+-- `podcast_order:<uuid>` key sits at exactly 50 chars, and future
+-- per-feed/per-collection preference keys must not be forced to
+-- payload-stuff by a column width. Widening a varchar is metadata-only
+-- (no table rewrite); idempotent on every boot.
+ALTER TABLE reader_settings ALTER COLUMN setting_key TYPE VARCHAR(120);
+
 CREATE INDEX IF NOT EXISTS idx_reader_settings_user ON reader_settings(user_id);
 
 -- PDF bookmarks (custom user bookmarks)