fix(pb-init): use python3 for JSON parsing in ensure_field; add heartbeat_at fields

The sed-based collection id and fields extraction was greedy and broke on
collections with multiple fields (grabbed the last field id instead of the
top-level collection id → PATCH to wrong URL → 404).

Rewrite ensure_field to use python3 for reliable JSON parsing. Also adds the
missing heartbeat_at (date) field to scraping_tasks and audio_jobs which was
never applied on the initial deploy because the bug prevented the PATCH.

scripts/pb-init.sh

@@ -98,22 +98,34 @@ ensure_field() {
     -H "Authorization: Bearer $TOKEN" \
     "$PB_URL/api/collections/$COLL" 2>/dev/null)
 
-  # Check if the field already exists (look for "name":"<FIELD_NAME>" in the fields array)
-  if echo "$SCHEMA" | grep -q "\"name\":\"$FIELD_NAME\""; then
+  # Use python3 to reliably parse the JSON schema.
+  PARSED=$(echo "$SCHEMA" | python3 -c "
+import sys, json
+try:
+    d = json.load(sys.stdin)
+    fields = d.get('fields', [])
+    exists = any(f.get('name') == '$FIELD_NAME' for f in fields)
+    print('exists=' + str(exists))
+    print('id=' + d.get('id', ''))
+    if not exists:
+        fields.append({'name': '$FIELD_NAME', 'type': '$FIELD_TYPE'})
+        print('fields=' + json.dumps(fields))
+except Exception as e:
+    print('error=' + str(e))
+" 2>/dev/null)
+
+  if echo "$PARSED" | grep -q "^exists=True"; then
     log "field $COLL.$FIELD_NAME already exists — skipping"
     return
   fi
 
-  COLLECTION_ID=$(echo "$SCHEMA" | sed 's/.*"id":"\([^"]*\)".*/\1/')
-  if [ -z "$COLLECTION_ID" ] || [ "$COLLECTION_ID" = "$SCHEMA" ]; then
+  COLLECTION_ID=$(echo "$PARSED" | grep "^id=" | sed 's/^id=//')
+  if [ -z "$COLLECTION_ID" ]; then
     log "WARNING: could not get id for collection $COLL — skipping ensure_field"
     return
   fi
 
-  # Extract current fields array and append the new field before the closing bracket.
-  CURRENT_FIELDS=$(echo "$SCHEMA" | sed 's/.*"fields":\(\[.*\]\).*/\1/')
-  TRIMMED=$(echo "$CURRENT_FIELDS" | sed 's/]$//')
-  NEW_FIELDS="${TRIMMED},{\"name\":\"${FIELD_NAME}\",\"type\":\"${FIELD_TYPE}\"}]"
+  NEW_FIELDS=$(echo "$PARSED" | grep "^fields=" | sed 's/^fields=//')
   PATCH_BODY="{\"fields\":${NEW_FIELDS}}"
 
   STATUS=$(curl -s -o /dev/null -w "%{http_code}" \