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/handlers/collections.go

@@ -86,10 +86,10 @@ func parseRequestFilterTree(payload any) ([]byte, error) {
 }
 
 type UpdateCollectionRequest struct {
-	Name            string                 `form:"name" json:"name" validate:"required"`
+	Name string `form:"name" json:"name" validate:"required"`
 	// Smart-collection match semantics (item 2.4): "any" (OR) | "all"
 	// (AND). Empty/invalid keeps the stored value.
-	RuleMatchMode   string                 `form:"rule_match_mode" json:"rule_match_mode"`
+	RuleMatchMode string `form:"rule_match_mode" json:"rule_match_mode"`
 	// The unified filter tree (item 3.2): when present the collection's
 	// smart membership runs through the qf engine. INVALID trees are a
 	// 400 (ParseFilterTree at the handler); a tree supersedes
@@ -547,6 +547,88 @@ func (h *CollectionHandler) UpdateCollection(c *echo.Context) error {
 	return c.JSON(http.StatusCreated, response)
 }
 
+// ValidChainOrders — the collection chain's listening-order vocabulary
+// (plan §10 R2: a collection's playback order is CURATED on the
+// collection, not a player-side knob). Mirrors OrderEpisodes' orders;
+// "" means native (the pre-chain-order default).
+var ValidChainOrders = map[string]bool{"": true, "native": true, "newest": true, "oldest": true, "reverse": true}
+
+// CollectionChainOrder resolves a collection row's curated chain order
+// from its view_settings JSONB — "" (native) when absent/unknown. The
+// episode chain producer routes OrderEpisodes through this; the
+// view-settings PUT writes the key (SetCollectionChainOrder).
+func CollectionChainOrder(viewSettings []byte) string {
+	if len(viewSettings) == 0 {
+		return ""
+	}
+	var vs map[string]interface{}
+	if json.Unmarshal(viewSettings, &vs) != nil {
+		return ""
+	}
+	if s, ok := vs["chain_order"].(string); ok && ValidChainOrders[s] {
+		return s
+	}
+	return ""
+}
+
+// SetCollectionChainOrder — PUT /api/collections/:id/view-settings with
+// {"chain_order": "..."}: merges the key into the collection's stored
+// view_settings (the playlist toggles share that JSONB — a blind write
+// would clobber them). The episode chain producer
+// (buildSmartCollectionSequenceContext) honors it.
+func (h *CollectionHandler) SetCollectionChainOrder(c *echo.Context) error {
+	collectionID, err := uuid.Parse(c.Param("id"))
+	if err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid collection id"})
+	}
+
+	var req struct {
+		ChainOrder string `json:"chain_order"`
+	}
+	if err := c.Bind(&req); err != nil {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid request"})
+	}
+	if !ValidChainOrders[req.ChainOrder] {
+		return c.JSON(http.StatusBadRequest, map[string]string{
+			"error": "chain_order must be one of: native, newest, oldest, reverse"})
+	}
+
+	collection, err := h.collectionService.GetCollection(c.Request().Context(), collectionID)
+	if err != nil {
+		return c.JSON(http.StatusNotFound, map[string]string{"error": "collection not found"})
+	}
+
+	settings := map[string]interface{}{}
+	if len(collection.ViewSettings) > 0 {
+		if uerr := json.Unmarshal(collection.ViewSettings, &settings); uerr != nil {
+			return c.JSON(http.StatusInternalServerError, map[string]string{"error": "corrupt view_settings"})
+		}
+	}
+	if req.ChainOrder == "" || req.ChainOrder == "native" {
+		// Both spellings mean the default — drop the key entirely so the
+		// stored doc stays clean.
+		delete(settings, "chain_order")
+	} else {
+		settings["chain_order"] = req.ChainOrder
+	}
+	blob, err := json.Marshal(settings)
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+	if err := h.db.UpdateCollectionViewSettings(c.Request().Context(), database.UpdateCollectionViewSettingsParams{
+		ID:           pgtype.UUID{Bytes: collectionID, Valid: true},
+		ViewSettings: blob,
+	}); err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
+	}
+
+	return c.JSON(http.StatusOK, map[string]interface{}{
+		"id":            uuid.UUID(collection.ID.Bytes).String(),
+		"chain_order":   req.ChainOrder,
+		"view_settings": settings,
+	})
+}
+
 func (h *CollectionHandler) DeleteCollection(c *echo.Context) error {
 	collectionID, err := uuid.Parse(c.Param("id"))
 	if err != nil {