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/create_collection.md

@@ -23,7 +23,6 @@ Creating a collection whose name already exists for the user returns **409**.
   "color": "#00FF00",
   "icon": "📖",
   "collection_type": "static",
-  "auto_assign_rules": [],
   "view_settings": { "kobo": { "view_mode": "grid" } }
 }
 ```
@@ -39,7 +38,6 @@ Creating a collection whose name already exists for the user returns **409**.
   "color": "#00FF00",
   "icon": "📖",
   "collection_type": "static",
-  "auto_assign_rules": null,
   "view_settings": { "kobo": { "view_mode": "grid" } },
   "book_count": 0,
   "created_at": "2026-10-24 13:57:01.81234 +0000 UTC"
@@ -57,20 +55,9 @@ RFC3339. Treat it as display text; do not parse it as an ISO timestamp.
 | description | string | no | |
 | color | string | no | hex color, e.g. `"#FF5733"` |
 | icon | string | no | emoji |
-| collection_type | string | no | `"static"` (default) or `"smart"`; also implied by non-empty `auto_assign_rules` |
-| auto_assign_rules | array | no | rule objects, see below |
+| collection_type | string | no | `"static"` (default) or `"smart"`; also implied by a `filter_tree` |
 | view_settings | object | no | per-device display preferences |
-
-### Auto-assign rule object
-
-| Field | Type | Required | Notes |
-| ----- | ---- | -------- | ----- |
-| id | string | no | rule id |
-| field | string | yes | one of `genre`, `series`, `author`, `language`, `publisher`, `copyright_year`, `tags` |
-| operator | string | yes | one of `equals`, `not_equals`, `contains`, `not_contains`, `starts_with`, `ends_with`, `greater_than`, `less_than` |
-| value | string | yes | value to compare against |
-| priority | number | no | 1-10; higher values take precedence |
-| enabled | boolean | no | absent = enabled (rules written before this field existed keep matching) |
+| filter_tree | object | no | the unified filter tree (qf DSL); presence makes the collection smart |
 
 ## Errors