refactor(form-builder): strict validator + drop form_fields.conditional_logic JSON column

WS-5c commit 3 of 4. FormRequests (Store/Update) now reject bad
conditional_logic trees at the HTTP boundary — the `after()` hook
unwraps the `show_when` envelope, normalises legacy `{all|any: [...]}`
group shape to the service's internal form, and delegates to
`FormFieldConditionalLogicService::assertSpecsValid()`. Unknown
operators, root conditions, empty groups, and unknown field_slug
references produce a 422 with a readable error before any write.

`form_fields.conditional_logic` JSON column dropped. FormField model
`$fillable` and `$casts` no longer mention the column; factory default
no longer writes `null` to it. Snapshot fixtures in the dev seeder and
the legacy-forms migration command keep `conditional_logic` in their
snapshot JSON shape — that's the schema_snapshot contract, not the DB
column.

FormFieldController now maps InvalidConditionalLogicSpecException to
422 alongside FrozenSchemaException / CyclicDependencyException.

Rollback path: roll back WS-5c commits 1–3 together. Partial rollback
(drop-column reversed but backfill still applied) is not a supported
state — matching the WS-5a/b precedent on the family's full-rollback
contract.

Tests: 6 new (strict FormRequest rejection cases + JSON-column drop
assertion). Rollback step counts in WS-5a/b migration tests bumped +1
for the drop_conditional_logic_json_column migration. Baseline
1142 → 1148 green (3085 → 3099 assertions).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/database/seeders/FormBuilderDevSeeder.php

@@ -100,7 +100,7 @@ final class FormBuilderDevSeeder
                     'is_filterable' => $field['is_filterable'] ?? false,
                     'is_pii' => $field['is_pii'] ?? false,
                     'binding' => null, // Pattern B — snapshot embeds null for form-owned fields.
-                    'conditional_logic' => null,
+                    'conditional_logic' => null, // snapshot shape: null for fields without conditional logic
                     'translations' => null,
                     'value_storage_hint' => $field['type']->recommendedValueStorageHint()->value,
                     'sort_order' => $sortOrder + 1,