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/lib/api.ts

@@ -738,6 +738,7 @@ interface RawAckRow {
   received_count: number;
   ack_code: "A" | "E" | "R" | "P";
   parsed_at: string;
+  patient_control_number?: string | null;
 }
 
 function mapAck(row: RawAckRow): Ack {
@@ -749,6 +750,7 @@ function mapAck(row: RawAckRow): Ack {
     receivedCount: row.received_count,
     ackCode: row.ack_code,
     parsedAt: row.parsed_at,
+    patientControlNumber: row.patient_control_number ?? null,
   };
 }