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/docs/schema.go

@@ -49,7 +49,7 @@ var Schemas = map[string]Schema{
 		Response: json.RawMessage(`{
 			"id": "", "user_id": "", "name": "", "description": "",
 			"color": "", "icon": "", "collection_type": "",
-			"auto_assign_rules": null, "view_settings": {},
+			"view_settings": {},
 			"book_count": 0, "created_at": ""
 		}`),
 	},
@@ -62,7 +62,7 @@ var Schemas = map[string]Schema{
 	"GET /api/collections/:id": {
 		Response: json.RawMessage(`{
 			"id": "", "user_id": "", "name": "", "description": "",
-			"color": "", "icon": "", "auto_assign_rules": null,
+			"color": "", "icon": "",
 			"view_settings": {}, "books": [], "book_count": 0,
 			"is_smart": false, "up_next": [], "created_at": ""
 		}`),
@@ -71,7 +71,7 @@ var Schemas = map[string]Schema{
 		Request: handlers.UpdateCollectionRequest{},
 		Response: json.RawMessage(`{
 			"id": "", "user_id": "", "name": "", "description": "",
-			"color": "", "icon": "", "auto_assign_rules": null,
+			"color": "", "icon": "",
 			"view_settings": {}, "created_at": ""
 		}`),
 	},
@@ -609,15 +609,6 @@ var Schemas = map[string]Schema{
 			"id": "", "chain_order": "", "view_settings": {}
 		}`),
 	},
-	"PUT /api/collections/:id/rules/:ruleId": {
-		// Handler is SetRuleEnabled: toggle only, request {enabled required},
-		// response {collection_id, is_smart}.
-		Request: json.RawMessage(`{"enabled": false}`),
-		Response: json.RawMessage(`{
-			"collection_id": "", "is_smart": false
-		}`),
-	},
-
 	// --- Beam batch 7: scanner ---
 	"POST /api/scanner/scan": {
 		Request: handlers.ScanLibraryRequest{},