feat(migrator): streaming batch approach for large schema migrations

- Add `batch_size` config to control subjects fetched per batch (default: 100)
- Add `workers` config for parallel sync within batches (default: 10)
- Stream schemas: fetch batch → sync batch → report progress → repeat
- Sample first batch to detect schema references
- Only sort subjects by schema ID when references are detected
- Force sequential processing when references exist to preserve ordering
- Real-time progress logging with synced/skipped counts

Ordering guarantees:
- If references detected: subjects sorted by schema ID, sequential processing
- If no references: parallel processing safe with fixed IDs (translate_ids=false)

Memory optimization:
- Only hold one batch in memory at a time
- Avoid 2x API calls in common case (no references)
- Tracking maps grow with synced schemas (unavoidable for deduplication)

Author: vuldin

docs/modules/components/pages/outputs/redpanda_migrator.adoc

@@ -53,6 +53,8 @@ output:
       translate_ids: false
       normalize: false
       strict: false
+      workers: 10
+      batch_size: 100
     consumer_groups:
       enabled: true
       interval: 1m
@@ -139,6 +141,8 @@ output:
       translate_ids: false
       normalize: false
       strict: false
+      workers: 10
+      batch_size: 100
     consumer_groups:
       enabled: true
       interval: 1m
@@ -1425,6 +1429,24 @@ Error on unknown schema IDs. Only relevant when translate_ids is true. When fals
 
 *Default*: `false`
 
+=== `schema_registry.workers`
+
+Number of parallel workers for schema sync operations. Higher values improve throughput for large schema counts.
+
+
+*Type*: `int`
+
+*Default*: `10`
+
+=== `schema_registry.batch_size`
+
+Number of subjects to fetch and sync per batch. Schemas are streamed in batches rather than fetched all at once, reducing memory usage and providing real-time progress for large migrations.
+
+
+*Type*: `int`
+
+*Default*: `100`
+
 === `consumer_groups`
 
 Sorry! This field is missing documentation.