fix(acks): make 999 source_batch_id unique per file + surface PCN

Gainwell's MFT ships every 999 with the same ISA interchange
control number (`000000001`) and one 999 ack covers a whole
batch, not a single claim — so the AK2 set_control_number
(patient_control_number) is the same for the ~96 999s in a
batch. With the old synthetic-id formula (`999-{icn}`), all 385
daily acks collapsed onto a single row the operator couldn't
distinguish.

The new formula is `999-{pcn}-{filename_hash8}` (or
`999-{icn}-{filename_hash8}` for envelope-only 999s without an
AK2). The PCN gives the operator a human-readable handle to the
claim batch; the 8-char hash of the inbound filename guarantees
uniqueness within a batch. Fits in the VARCHAR(32) source_batch_id
column (max 22 chars).

Also surface `patient_control_number` in /api/acks list response
(extracted from raw_json's set_responses[0].set_control_number)
and in the Acks UI as the primary label, with the synthetic id
shown dimmed after a middle dot. The detail endpoint already
exposed raw_json for the full 999 parse tree.

src/pages/Acks.tsx

@@ -365,7 +365,18 @@ export function Acks() {
                                 {a.id}
                               </TableCell>
                               <TableCell className="font-mono text-[12px] text-muted-foreground truncate max-w-[180px]">
-                                {a.sourceBatchId}
+                                {a.patientControlNumber ? (
+                                  <>
+                                    <span className="text-foreground">
+                                      {a.patientControlNumber}
+                                    </span>
+                                    <span className="text-muted-foreground/60">
+                                      {" "}· {a.sourceBatchId}
+                                    </span>
+                                  </>
+                                ) : (
+                                  a.sourceBatchId
+                                )}
                               </TableCell>
                               <TableCell className="text-right display mono text-[hsl(var(--success))]">
                                 {a.acceptedCount}