feat(podcasts): the episodes authoring door + curated chain order

"Newest episodes from my selected podcasts" is now buildable in the UI,
end to end — the user could not find ANY podcast-related filter surface
because none existed: the New Collection modal could only create
book-scope smart collections, and the engine's episodes scope (item 3.1)
had no authoring door.

The scope door:
- The create modal's Smart card gains a Rules-scope segmented control
  (📚 Books / 🎧 Podcast episodes); the copy no longer claims smart
  means "match books by rules". Choosing Episodes seeds an EMPTY
  episodes tree ({"scope":"episodes","mode":"all","children":[]}) so the
  collection is born episode-scoped: the editor opens with the episode
  vocabulary (feed/downloaded/played/published date) and membership
  runs the episode engine from the first rule.
- ParseFilterTree now accepts an EMPTY ROOT (nested empty groups stay
  invalid) — the seed's legal shape. The root arrives at depth 1 by
  call convention, hence the depth>1 exemption.
- The composers define the empty-tree semantics: zero children matches
  NOTHING (ComposedLiveMedia/ComposedCount guards), mirroring
  EvaluateSmartCollection's zero-enabled-rules convention. Without the
  guard the composed query degrades to the bare scope head and a fresh
  collection would swallow every episode in scope — pinned by
  TestEpisodeCollectionsEmptySeedMatchesNothing (envelope book_count 0)
  and TestEmptyRootTreeSeedsEpisodesScope.

The curated chain order (plan §10 R2, user need: the briefing plays
NEWEST-first):
- view_settings.chain_order ("native"/"newest"/"oldest"/"reverse")
  stored on the collection — curated content, not a player-side knob.
- PUT /api/collections/:id/view-settings merges the key into the
  stored JSONB (a blind write would clobber the playlist toggles).
- buildSmartCollectionSequenceContext routes OrderEpisodes through
  handlers.CollectionChainOrder; the detail header's Play-as-Sequence
  gains the order select next to it (PlayCollectionButton), which
  PUTs and reloads. Pinned by chain_follows_the_curated_order
  (reverse flips the chain indexes and hops).

Caught by the new test, a latent clobber fixed: UpdateCollection
treated a NIL view_settings payload as "write NULL" — any edit that
carried no settings dropped every stored key (chain_order, playlist
toggles from API clients, imported provenance). Nil now means
untouched, the same convention filterTree already had.

Composer guards also cover ComposedCount, so the builder's live count
reads the same semantics. Tests: collectionChainOrder unit matrix,
empty-root parse unit pin, two integration pins (seed matches nothing;
curated order drives the chain and survives whole-map edits).

internal/services/collection_service.go

@@ -189,6 +189,17 @@ func (s *CollectionService) UpdateCollection(ctx context.Context, collectionID u
 				viewSettings = merged
 			}
 		}
+		// Same convention as filterTree: a NIL view_settings payload
+		// means untouched — keep the stored doc. Without this, an edit
+		// that carries no settings wrote NULL over the row and silently
+		// dropped every stored key (chain_order, imported provenance);
+		// the 2026-10-31 chain-order test pinned exactly this clobber.
+		if viewSettings == nil && len(existing.ViewSettings) > 0 {
+			var stored map[string]any
+			if json.Unmarshal(existing.ViewSettings, &stored) == nil && len(stored) > 0 {
+				viewSettings = stored
+			}
+		}
 	}
 	if !ValidRuleMatchMode(ruleMatchMode) {
 		ruleMatchMode = existingMode
@@ -493,8 +504,6 @@ func IsCollectionSmart(coll database.Collections) bool {
 	return len(rules) > 0
 }
 
-
-
 // EvaluateSmartMembership is THE single smart evaluator (item 3.2):
 // tree-bearing collections evaluate through the unified filter engine;
 // legacy rule arrays keep per-item evaluation until rewritten. One