refactor(collections): retire the legacy rules surface — tree-only storage

The pre-tree rules machinery is deleted: no row ever carried a rules
array or a non-default match mode (verified in the dev DB), the web
never read the envelope keys, and the Android app consumes none of it.
Dead code gets no documentation.

Removed:
- PUT /api/collections/:id/match-mode — endpoint, route, and the
  UpdateCollectionMatchMode query. The tree's root mode owns any|all.
- auto_assign_rules / rule_match_mode from every envelope
  (create/list/detail/update), from the create/update request shapes,
  and from the schema: both columns DROPPED (schema.sql carries
  idempotent DROP COLUMN IF EXISTS heals for existing DBs; sqlc
  regenerated).
- The rules CRUD service layer (AddRule/SetRuleEnabled/RemoveRule +
  validateRule + UpdateCollectionRulesJSON), ParseRules /
  EnabledRules / IsSmartCollection, RulesToFilterTree (+ the SSR
  preload branch), the legacy branch of EvaluateSmartMembership, and
  SmartEvalOptions.MatchMode. IsCollectionSmart = tree presence;
  create-path isSmart = collection_type=smart OR a filter_tree.

Kept (live consumers): POST /api/collections/preview still evaluates
rules payloads in memory (the custom-section builder's preview), and
include_auto stays on the list (the Android app sends it).

The custom-section builder's SAVE was already broken — its
stringified auto_assign_rules payload failed request binding (probed
live: 400 invalid request). Create now rejects that payload loudly
with a 400 naming the retirement, instead of silently creating a
hollow static section; custom-section-builder.md carries the flag.
The builder's tree migration remains the flagged follow-up.

Test-side (asymmetry rule — deleted behavior, deleted tests):
TestPodcastSmartCollections rewritten onto trees (mode lives in the
tree root), match-mode subtests and tree_wins_over_legacy_rules
deleted, TestRulesToFilterTree deleted, contract-pin envelope keys
retracted per the contract_pins 2026-10-31 retirement entry. Mirrors
hand-synced: openapi.yaml, bruno create/update, docs/api/*.md,
docs/schema.go, api.d.ts. Full container suite green (155s).

internal/services/dashboard_service.go

@@ -292,56 +292,15 @@ func (s *DashboardService) getUserCollectionItems(ctx context.Context, coll data
 	}
 
 	// The query filters excluded rows in SQL and embeds the media row,
-	// so the items are MediaItems as-is.
-	var manualNonExcluded []database.MediaItems
+	// so the items are MediaItems as-is. (The legacy auto-assign branch
+	// retired 2026-10-31 with the rules-array storage: smart user
+	// collections evaluate on their detail page; sections render their
+	// stored membership.)
+	items := make([]database.MediaItems, 0, len(manualItems))
 	for _, row := range manualItems {
-		manualNonExcluded = append(manualNonExcluded, row.MediaItems)
+		items = append(items, row.MediaItems)
 	}
-
-	var autoItems []database.MediaItems
-	if len(coll.AutoAssignRules) > 0 {
-		var rules []Rule
-		if err := json.Unmarshal(coll.AutoAssignRules, &rules); err == nil && len(rules) > 0 {
-			allLibraryItems, err := s.db.GetLibraryItems(ctx, libraryID)
-			if err == nil {
-				for _, item := range allLibraryItems {
-					alreadyInCollection := false
-					for _, manualItem := range manualNonExcluded {
-						manualUUID := uuid.UUID(manualItem.ID.Bytes)
-						itemUUID := uuid.UUID(item.ID.Bytes)
-						if manualUUID == itemUUID {
-							alreadyInCollection = true
-							break
-						}
-					}
-					if alreadyInCollection {
-						continue
-					}
-
-					// Legacy dashboard auto-assign path: no viewer progress
-					// loaded here (the dashboard's own hide_finished handles
-					// queue semantics) — played rules evaluate unplayed, honestly.
-					evaluations := s.collectionService.EvaluateRules(item, 0, rules)
-					for _, eval := range evaluations {
-						if eval.Matches {
-							autoItems = append(autoItems, item)
-							break
-						}
-					}
-				}
-			}
-		}
-	}
-
-	var finalItems []database.MediaItems
-	finalItems = append(finalItems, manualNonExcluded...)
-	finalItems = append(finalItems, autoItems...)
-
-	if len(finalItems) > limit {
-		finalItems = finalItems[:limit]
-	}
-
-	return finalItems, nil
+	return items, nil
 }
 
 func (s *DashboardService) GetDashboardPreferences(ctx context.Context, userID uuid.UUID, libraryID pgtype.UUID) (database.UserDashboardPreferences, error) {