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/router/frontend.go

@@ -569,7 +569,6 @@ func registerFrontendRoutes(cfg *Config) {
 					col, services.SmartEvalOptions{
 						LibraryID:    libScope,
 						VisibleIDs:   visibleIDs,
-						MatchMode:    col.RuleMatchMode,
 						ViewerUserID: func() pgtype.UUID {
 							uid, _ := uuid.Parse(user.ID)
 							return uuidToPGType(uid)
@@ -702,9 +701,9 @@ func registerFrontendRoutes(cfg *Config) {
 		}
 
 		// The editor's vocabulary + prefill (item 3.4): the unified
-		// builder in the collection's scope. A stored tree renders as-is;
-		// a legacy rules array preloads as its CONVERTED tree
-		// (RulesToFilterTree) — the upgrade lands at the first Apply.
+		// builder in the collection's scope. A stored tree renders
+		// as-is. (The legacy rules-array preload retired 2026-10-31
+		// with the rules-array storage — the tree is the only form.)
 		scope := "books"
 		treeJSON := "{}"
 		if len(collection.FilterTree) > 0 {
@@ -714,13 +713,6 @@ func registerFrontendRoutes(cfg *Config) {
 					treeJSON = string(b)
 				}
 			}
-		} else if rules := cfg.CollectionHandler.CollectionService().ParseRules(collection.AutoAssignRules); len(rules) > 0 {
-			if t, cerr := services.RulesToFilterTree(rules, collection.RuleMatchMode); cerr == nil {
-				scope = t.FilterScope()
-				if b, merr := json.Marshal(t); merr == nil {
-					treeJSON = string(b)
-				}
-			}
 		}
 		fieldsPayload, merr := json.Marshal(services.FilterFieldsPayload(scope))
 		if merr != nil {