S2a: purge legacy Form Builder PHP code and routes

dev-docs/ARCH-FORM-BUILDER.md

@@ -2955,6 +2955,30 @@ Every contract in §31.1-31.8 has a corresponding integration test in
 Tests are part of CI. Contract changes require test updates (and this
 ARCH section update) before merge.
 
+### 31.10 Tag sync integration (BACKLOG FORM-02)
+
+Replaces the S1-era `TagSyncService` that read the legacy
+`person_field_values` table. Purged in S2a; rebuilt in S2b/S2c against
+the new FormBuilder.
+
+**Trigger:** `FormSubmissionSubmitted` event (ARCH §17.1) OR explicit
+call from `PersonController::approve()` after status transitions to
+`approved`.
+
+**Listener:** `SyncTagPickerValuesToUserTagsListener` — for the given
+submission, finds all `form_values` whose field has
+`field_type=TAG_PICKER`, and upserts rows into `user_organisation_tags`
+with `source=self_reported`, respecting `person.user_id` (skip if null).
+Only syncs to the subject person's user account.
+
+**Failure mode:** log at warning level; never throws into the submission
+lifecycle. Reason: a tag-sync failure must not block registration.
+
+**Call site removed in S2a:** `PersonController::approve()` and
+`PersonIdentityService::syncRegistrationTags()` used to call
+`TagSyncService::syncFromRegistration($person)` directly. The rebuilt
+flow is listener-driven — no direct service injection required.
+
 ---
 
 ## End of ARCH v1.2