test(pins): widen whitelist for the episode-collections JSON shapes (item 2 series)

TestContractPin_CollectionsJSON tripped on the additive keys the
episode-collections work introduced — the tripwire worked exactly as
designed (additive keys must be whitelisted here AND in
docs/contract_pins.md first).

- Collections envelope gains rule_match_mode (item 2.4): the
  smart-collection match semantics on GET /api/collections/:id.
- BookInfo gains four STABLE keys (item 2.3): is_episode,
  feed_title, download_state, hidden — dropped omitempty so every
  card surface emits the same key set (the viewer_percentage
  precedent; adapters that predate episode awareness emit zero
  values).

docs/contract_pins.md changelog entry written. Nothing removed,
nothing renamed.

cmd/server/tests/contract_pins_test.go

@@ -459,6 +459,8 @@ var bookInfoKeys = []string{
 	"media_item_id", "title", "author", "cover_image_path",
 	"has_conflict", "format_group", "badge",
 	"viewer_percentage", "library_name",
+	// Episode-collections extras (2.3 widening, every surface emits):
+	"is_episode", "feed_title", "download_state", "hidden",
 }
 
 func collectionURL(setup *TestServerSetup, id string) string {
@@ -500,6 +502,7 @@ func TestContractPin_CollectionsJSON(t *testing.T) {
 		"id", "user_id", "name", "description", "color", "icon",
 		"auto_assign_rules", "view_settings", "books", "book_count",
 		"is_smart", "created_at", "up_next",
+		"rule_match_mode", // item-2.4 widening (docs/contract_pins.md)
 	}
 
 	t.Run("user collection with library_id (badged branch)", func(t *testing.T) {

docs/contract_pins.md

@@ -468,3 +468,16 @@ never-queued episodes) is stamped from the `podcast_queue_seq` SEQUENCE
 at enqueue time so claim order = enqueue order (the effective listening
 order). Raw-row pins (TestContractPin_MediaListBothBranches →
 rawRowKeys) whitelist the key. Nothing removed, nothing renamed.
+
+## 2026-10-30 — episode collections (additive widening, plan item 2/2.3/2.4)
+
+Two shapes widen, both ADDITIVE — nothing removed, nothing renamed:
+
+- **Collections envelope** (`GET /api/collections/:id` → envelopeKeys)
+  gains `rule_match_mode` (smart-collection match semantics, 'any'
+  legacy default | 'all'). Static collections emit the default value.
+- **BookInfo** (every card surface → bookInfoKeys) gains four STABLE
+  keys — `is_episode`, `feed_title`, `download_state`, `hidden` — the
+  episode-collections extras. Adapters that predate episode awareness
+  emit their zero values (`false`/`""`), matching the
+  viewer_percentage precedent (nil-representable, key always present).

internal/handlers/cards.go

@@ -50,11 +50,14 @@ type BookInfo struct {
 	// LEFT JOIN missed under RLS (unsubscribed episode, or a
 	// family-lens hidden book) — the row renders as an "unavailable"
 	// hint instead of silently vanishing. Adapters that don't load
-	// visibility leave Hidden false (cards stay fully rendered).
+	// visibility leave Hidden false (cards stay fully rendered). The
-	IsEpisode     bool   `json:"is_episode,omitempty"`
+	// four keys are NOT omitempty: every BookInfo surface emits the
-	FeedTitle     string `json:"feed_title,omitempty"`
+	// same stable key set (the viewer_percentage precedent —
-	DownloadState string `json:"download_state,omitempty"`
+	// contract-pin widening, docs/contract_pins.md 2026-10-30).
-	Hidden        bool   `json:"hidden,omitempty"`
+	IsEpisode     bool   `json:"is_episode"`
+	FeedTitle     string `json:"feed_title"`
+	DownloadState string `json:"download_state"`
+	Hidden        bool   `json:"hidden"`
 }
 
 // LibraryTypeMap builds libraryID→library-type-name for badge resolution