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.

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).
-	IsEpisode     bool   `json:"is_episode,omitempty"`
-	FeedTitle     string `json:"feed_title,omitempty"`
-	DownloadState string `json:"download_state,omitempty"`
-	Hidden        bool   `json:"hidden,omitempty"`
+	// visibility leave Hidden false (cards stay fully rendered). The
+	// four keys are NOT omitempty: every BookInfo surface emits the
+	// same stable key set (the viewer_percentage precedent —
+	// contract-pin widening, docs/contract_pins.md 2026-10-30).
+	IsEpisode     bool   `json:"is_episode"`
+	FeedTitle     string `json:"feed_title"`
+	DownloadState string `json:"download_state"`
+	Hidden        bool   `json:"hidden"`
 }
 
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