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).

docs/contract_pins.md

@@ -545,3 +545,35 @@ Test-side: TestCollectionsRLS pins the cross-user matrix end-to-end;
 fixture writes to non-system collections run as the row's OWNER
 (scopeExecAs/ownerSeedCtx) because the admin write-escape covers
 system collections only.
+
+## 2026-10-31 — legacy collection rules surface RETIRED (tree-only storage)
+
+The pre-tree rules machinery is gone — no production row ever carried a
+rules array or a non-default match mode (verified in the dev DB), the
+web never reads the envelope keys, and the Android app consumes none of
+it. Deletions:
+
+- `PUT /api/collections/:id/match-mode` — endpoint retired (it had no
+  doc page). The tree's root `mode` owns any|all semantics now.
+- Envelope: `auto_assign_rules` and `rule_match_mode` are REMOVED from
+  the create/list/detail/update payloads. This entry retracts the
+  2026-10-30 pins that listed them. `collection_type` and `is_smart`
+  stay.
+- Request: `auto_assign_rules`/`rule_match_mode` are no longer accepted
+  on create/update. A create carrying `auto_assign_rules` answers 400
+  naming the retirement — the custom-section builder's legacy save
+  (already failing to bind, verified live) fails loudly instead of
+  silently creating a hollow static section.
+- Storage: the `collections.auto_assign_rules` and
+  `collections.rule_match_mode` columns are DROPPED (idempotent
+  `DROP COLUMN IF EXISTS` in schema.sql).
+- Service: the rules CRUD layer (AddRule/SetRuleEnabled/RemoveRule +
+  validateRule + the UpdateCollectionRulesJSON query),
+  ParseRules/EnabledRules/IsSmartCollection, RulesToFilterTree (+ the
+  SSR preload branch), and the legacy branch of EvaluateSmartMembership
+  are deleted. IsCollectionSmart = tree presence.
+
+KEPT (live consumers): `POST /api/collections/preview` still accepts
+`rules` and evaluates them in memory (the custom-section builder's
+preview button); `include_auto` on the list stays (the Android app
+sends it). The builder's SAVE remains the flagged §12 tree migration.