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).

cmd/server/tests/contract_pins_test.go

@@ -500,10 +500,9 @@ func TestContractPin_CollectionsJSON(t *testing.T) {
 	// payload; docs/contract_pins.md changelog).
 	envelopeKeys := []string{
 		"id", "user_id", "name", "description", "color", "icon",
-		"auto_assign_rules", "view_settings", "books", "book_count",
+		"view_settings", "books", "book_count",
 		"is_smart", "created_at", "up_next",
-		"rule_match_mode", // item-2.4 widening (docs/contract_pins.md)
-		"filter_tree",     // item-3.2 widening (docs/contract_pins.md)
+		"filter_tree", // item-3.2 widening (docs/contract_pins.md)
 	}
 
 	t.Run("user collection with library_id (badged branch)", func(t *testing.T) {
@@ -566,11 +565,14 @@ func TestContractPin_CollectionsJSON(t *testing.T) {
 		assertArrayItemShape(t, "collection books[] (no library)", books, bookInfoKeys, bookInfoKeys)
 	})
 
-	t.Run("smart collection (rules branch)", func(t *testing.T) {
+	t.Run("smart collection (tree branch)", func(t *testing.T) {
 		resp := doReq(t, "POST", setup.Server.URL+"/api/collections", map[string]interface{}{
 			"name": fmt.Sprintf("Test Contract Smart %d", time.Now().UnixNano()),
-			"auto_assign_rules": []map[string]interface{}{
-				{"id": "rule-1", "field": "genre", "operator": "equals", "value": "Science Fiction", "priority": 5},
+			"filter_tree": map[string]interface{}{
+				"scope": "books", "mode": "any",
+				"children": []map[string]interface{}{
+					{"field": "genre", "op": "is", "values": []string{"Science Fiction"}},
+				},
 			},
 		}, setup.Token)
 		defer resp.Body.Close()