File upload pipeline: wire end-to-end via APIv4 File.create

Closes the file-upload gap. Files now actually land in CiviCRM (verified
empirically against the live Civi instance via spike scripts).

Spike findings (see scripts/spike-file-upload.mjs):
  - APIv4 Attachment is NOT exposed on this Civi
  - APIv4 File + EntityFile ARE exposed; File.create accepts inline
    base64 `content` and returns a usable file id
  - Custom file fields store the file id directly in the custom column,
    so EntityFile linkage is unnecessary for this use case
  - Round-trip via Contact.update + Contact.get .file_name join verified
    on a real org contact

Pipeline:

  Renderer (FileField) picks up onChange  →
    POST /api/upload (multipart) with file + cid + cs + fieldRef  →
      verifyChecksum, MIME allowlist + magic-byte sniff, 5 MB cap  →
        civi.File.create({ file_name, mime_type, content: base64 })  →
          returns { id, file_name }  →
            renderer stores in RHF state via setValue
  Form submit  →
    POST /api/submit (JSON) with the {id, file_name} value  →
      submit detects the file shape and writes the id as the value of
      the activity/contact custom field

File changes:

  app/api/upload/route.ts
    Replaced the 501 stub with the real File.create call. Comment
    documents that EntityFile linkage is intentionally skipped and that
    orphan cleanup is owned by a CiviCRM scheduled job.

  app/api/submit/route.ts
    For type:"file" values shaped as {id, file_name}, write the id as
    the custom field value (activity or contact, depending on the
    civiField / civiContactField the field declares).

  components/fields/FieldRenderer.tsx
    Replaced the bare <input type=file> register() with FileField, an
    upload-on-pick subcomponent. The native input is NOT register()'d:
    its FileList value was the original bug. FileField owns its
    uploading + error state and writes {id, file_name} via setValue on
    success. Submit is blocked upstream while uploads are in flight.

  components/StageSection.tsx, components/EngagementForm.tsx
    Thread setValue, cid, cs, and an onUploadStateChange callback
    through to FieldRenderer. EngagementForm tracks uploads-in-flight
    count; onSubmit refuses to submit while the count is > 0.

  config/form.ts
    Promotes Certificate_of_Incorporation from readonly to a real
    file field now that the pipeline works.

  app/api/data/route.ts
    Drops the readonly carveout that was only needed while the
    certificate was readonly.

  scripts/list-civi-entities.mjs (new)
    APIv4 entity probe + APIv3 attachment-API probe. Used to determine
    that File (not Attachment) was the right entity on this Civi.

  scripts/spike-file-upload.mjs (new)
    The actual end-to-end test that proved out the pipeline before
    wiring. Safe to re-run on any Civi instance during future audits.

Not in this change:
  - Orphan attachment cleanup (CiviCRM scheduled job, Civi admin scope)
  - Per-field MIME allowlists (single global list for v1)
  - S3 / presigned-URL path for >5 MB files (deferred; capped at 5 MB
    today to stay under Amplify Lambda's 6 MB sync payload limit)

app/api/submit/route.ts

@@ -120,10 +120,22 @@ async function runSubmit(cid: string, cs: string, values: Record<string, unknown
     const field = FIELD_BY_NAME.get(name);
     if (!field) continue;
     if (field.type === "readonly") continue; // never write read-only fields
+
+    // File fields: the renderer uploads to /api/upload on file-pick and
+    // stores {id, file_name} in form state. Submit only needs the id —
+    // that's what Civi stores in the custom column. If the user left a
+    // prior attachment alone, we receive the same prefill shape and
+    // still write the same id (no-op effectively).
+    let civiValue: unknown = value;
+    if (field.type === "file" && value && typeof value === "object" && !Array.isArray(value)) {
+      const v = value as { id?: unknown };
+      civiValue = typeof v.id === "number" || typeof v.id === "string" ? v.id : null;
+    }
+
     if (field.civiContactField) {
-      orgContactValues[field.civiContactField] = value;
+      orgContactValues[field.civiContactField] = civiValue;
     } else if (field.civiField) {
-      activityRecord[field.civiField] = value;
+      activityRecord[field.civiField] = civiValue;
     }
   }