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/developer/api/collections/update_collection.md

@@ -24,7 +24,6 @@ set `view_settings.hide_finished` / `view_settings.show_up_next` directly.
   "description": "Updated description",
   "color": "#33CCFF",
   "icon": "📚",
-  "auto_assign_rules": [],
   "view_settings": { "hide_finished": true },
   "hide_finished": ["true"],
   "show_up_next": ["true"]
@@ -41,7 +40,6 @@ set `view_settings.hide_finished` / `view_settings.show_up_next` directly.
   "description": "Updated description",
   "color": "#33CCFF",
   "icon": "📚",
-  "auto_assign_rules": null,
   "view_settings": { "hide_finished": true, "show_up_next": true },
   "created_at": "2026-10-24 13:57:01.81234 +0000 UTC"
 }
@@ -57,8 +55,8 @@ set `view_settings.hide_finished` / `view_settings.show_up_next` directly.
 | description | string | no | |
 | color | string | no | |
 | icon | string | no | |
-| auto_assign_rules | array | no | replaces the whole rule set (rule object shape: see Create Collection) |
 | view_settings | object | no | merged into stored settings |
+| filter_tree | object | no | replaces the stored filter tree (omitting it keeps the stored one) |
 | hide_finished | array | no | single-element string array `["true"]`/`["false"]`; merged into view_settings |
 | show_up_next | array | no | same shape as `hide_finished` |