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/database/querier.go

@@ -599,14 +599,6 @@ type Querier interface {
 	GetUserSettingByKey(ctx context.Context, arg GetUserSettingByKeyParams) ([]byte, error)
 	GetUserVisibleLibraries(ctx context.Context, userID pgtype.UUID) ([]GetUserVisibleLibrariesRow, error)
 	GetVisibleLibraryMediaCounts(ctx context.Context, userID pgtype.UUID) ([]GetVisibleLibraryMediaCountsRow, error)
-	// Smart ("dynamic") collections: rules are evaluated LIVE at render, so
-	// membership comes from the candidate book set instead of collection_items.
-	// The set is scoped to the requesting user's visible libraries (family
-	// lens enforced at the source) and live items only — the same filters the
-	// static collection paths apply. The viewer's progress rides along (plan
-	// item 2.4): the played/unplayed episode rule evaluates per-user, exactly
-	// like the hide_finished predicate (percentage >= 0.999 = finished).
-	GetVisibleLiveMediaForRules(ctx context.Context, arg GetVisibleLiveMediaForRulesParams) ([]GetVisibleLiveMediaForRulesRow, error)
 	HasRecentConflictResolution(ctx context.Context, arg HasRecentConflictResolutionParams) (bool, error)
 	// Soft-dismiss one row (ownership is checked by the caller against the
 	// device's user_id before calling this).
@@ -828,12 +820,6 @@ type Querier interface {
 	UnignoreUnlinkedBook(ctx context.Context, id pgtype.UUID) error
 	// Update collection
 	UpdateCollection(ctx context.Context, arg UpdateCollectionParams) (Collections, error)
-	// Smart-collection match semantics (plan item 2.4): flip any|all on its
-	// own channel — the rules page toggle writes exactly this column.
-	UpdateCollectionMatchMode(ctx context.Context, arg UpdateCollectionMatchModeParams) error
-	// Replace ONLY the auto_assign_rules document (rule CRUD: read-modify-write
-	// of the JSONB without touching metadata stale clients did not send).
-	UpdateCollectionRulesJSON(ctx context.Context, arg UpdateCollectionRulesJSONParams) (Collections, error)
 	// A2c §2.4: provenance + playlist flags on an existing collection without
 	// touching the rest of the row.
 	// (Historical bug, fixed 2026-10-30: this wrote `updated_at = NOW()` but