feat(dashboard): custom-section builder saves again — the flagged tree migration

Archaeology, prompted by the user asking when it broke: the builder's
save NEVER worked. The page was born (2026-02-19, a92bf99a) posting
auto_assign_rules as a JSON-encoded STRING into an endpoint that has
bound []Rule since its creation (2026-01-31, 6495cc2c), plus fields
(manual_book_ids, match_type, library_id, show_on_dashboard) the
endpoint never accepted. The preview button hit a real endpoint, so
the page looked alive while Save always failed at request binding.
Verified live before this change: the page's exact payload 400'd
"invalid request" against the unmodified server.

The fix adapts the builder's payload at write time — the legacy rules
vocabulary converts to a filter tree (RulesToFilterTree restored from
the retirement commit as the single write path, unit test restored
with it):

- match_type seeds the converted tree's root mode (any default).
- manual_book_ids attach to the created collection after create
  (unparsable ids skipped — the AddBooks leniency).
- show_on_dashboard persists (new create field).
- Converted trees validate under ParseFilterTree: an unhonorable rule
  is a 400 at write naming the problem, never a stored 500 at render.

Dashboard sections: smart user sections now evaluate their tree LIVE —
GetUserCollectionItems merges stored membership with the composed
engine's matches (deduped, library-scoped, capped at the section
limit). This exposed a REAL production wiring bug: every production
NewDashboardService call omitted the search service, so tree
evaluation through the dashboard could never have worked (the test
harness wired it correctly — the gap was production-only and latent,
because nothing in the dashboard evaluated trees before this
migration). The dashboard handler and both inline sites now receive
the search service, and evaluation failures log instead of silently
emptying a section.

Supersedes the same-day loud-400 stance for the builder's payload
(contract_pins migration entry); custom-section-builder.md documents
the save contract; the create_collection API doc gains the adapted
fields. New TestCustomSectionBuilderSave/BadRule pin the payload →
tree → dashboard-render path end to end. Full container suite green
(155s).

docs/developer/api/collections/create_collection.md

@@ -58,6 +58,10 @@ RFC3339. Treat it as display text; do not parse it as an ISO timestamp.
 | collection_type | string | no | `"static"` (default) or `"smart"`; also implied by a `filter_tree` |
 | view_settings | object | no | per-device display preferences |
 | filter_tree | object | no | the unified filter tree (qf DSL); presence makes the collection smart |
+| auto_assign_rules | string \| array | no | custom-section-builder payload — converted to `filter_tree` at write (see custom-section-builder doc) |
+| match_type | string | no | builder payload: `any` (default) or `all` — seeds the converted tree's root mode |
+| manual_book_ids | string\[\] | no | builder payload: attached to the collection after create |
+| show_on_dashboard | boolean | no | pins the collection as a dashboard section |
 
 ## Errors