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/collection_service_test.go

@@ -2,12 +2,9 @@ package services
 
 import (
 	"bookhoard/internal/database"
-	"encoding/json"
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgtype"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
 func TestEvaluateRules_Equality(t *testing.T) {
@@ -280,44 +277,7 @@ func TestEvaluateRules_PriorityBoost(t *testing.T) {
 	}
 }
 
-func TestRulesToFilterTree(t *testing.T) {
-	t.Run("episode_rules_map_mechanically", func(t *testing.T) {
-		rules := []Rule{
-			{ID: "1", Field: "feed", Operator: "in", Value: "feed-a, feed-b"},
-			{ID: "2", Field: "played", Operator: "equals", Value: "false"},
-			{ID: "3", Field: "downloaded", Operator: "equals", Value: "true"},
-			{ID: "4", Field: "published_within", Operator: "greater_than", Value: "7"},
-		}
-		tree, err := RulesToFilterTree(rules, RuleMatchAll)
-		require.NoError(t, err)
-		assert.Equal(t, RuleMatchAll, tree.Mode, "the collection's match mode becomes the tree mode")
-		assert.Equal(t, ScopeEpisodes, tree.Scope, "episode rules force the episodes scope")
-		require.Len(t, tree.Children, 4)
-		assert.Equal(t, FilterChild{Field: "feed", Op: OpIn, Values: []string{"feed-a", "feed-b"}}, tree.Children[0])
-		assert.Equal(t, FilterChild{Field: "played", Op: OpIsFalse}, tree.Children[1])
-		assert.Equal(t, FilterChild{Field: "downloaded", Op: OpIsTrue}, tree.Children[2])
-		assert.Equal(t, FilterChild{Field: "published_date", Op: OpWithinDays, Values: []string{"7"}}, tree.Children[3])
-		// The converted tree parses under the strict engine.
-		b, _ := json.Marshal(tree)
-		_, err = ParseFilterTree(string(b))
-		require.NoError(t, err, "converted tree must satisfy the engine grammar")
-	})
-
-	t.Run("book_rules_map_and_stay_books_scope", func(t *testing.T) {
-		rules := []Rule{
-			{Field: "genre", Operator: "equals", Value: "fantasy"},
-			{Field: "author", Operator: "contains", Value: "Voss"},
-			{Field: "copyright_year", Operator: "greater_than", Value: "1990"},
-			{Field: "tags", Operator: "starts_with", Value: "dyst"},
-		}
-		tree, err := RulesToFilterTree(rules, RuleMatchAny)
-		require.NoError(t, err)
-		assert.Equal(t, "", tree.Scope, "no episode rules → the books scope (legacy default)")
-		assert.Equal(t, RuleMatchAny, tree.Mode)
-		assert.Equal(t, FilterChild{Field: "genre", Op: OpIs, Values: []string{"fantasy"}}, tree.Children[0])
-		assert.Equal(t, FilterChild{Field: "copyright_year", Op: OpAtLeast, Values: []string{"1990"}}, tree.Children[2])
-		// Documented fallback: rules ops without an engine equivalent
-		// (starts_with/ends_with) become contains.
-		assert.Equal(t, OpContains, tree.Children[3].Op)
-	})
-}
+// TestRulesToFilterTree retired 2026-10-31 with the function it tested:
+// rules-array storage is gone, the filter tree is the only smart
+// membership. (Tests of deleted behavior are deleted — the asymmetry
+// rule.)