feat(config): move the upload-size ceiling under ingestion settings

Why:
- ADR-0017 names the bound INGESTION_MAX_UPLOAD_SIZE_MB; the code had it
  as APP_MAX_UPLOAD_SIZE_MB. Reconciled to match the ADR, grouped with the
  other three ADR-0017 bounds on IngestionSettings.

.env.example

@@ -9,7 +9,6 @@
 
 # Application
 APP_ENV=local
-APP_MAX_UPLOAD_SIZE_MB=25
 APP_READINESS_CHECK_TIMEOUT_SECONDS=2.0
 
 # Logging
@@ -37,6 +36,7 @@ MINIO_BUCKET=chatbot-source-files
 INGESTION_MAX_CONCURRENCY=4
 INGESTION_THREAD_POOL_SIZE=8
 INGESTION_TIMEOUT_SECONDS=120.0
+INGESTION_MAX_UPLOAD_SIZE_MB=25
 INGESTION_MAX_CHUNKS_PER_FILE=5000
 INGESTION_EMBED_BATCH_SIZE=128
 INGESTION_EMBED_CONCURRENCY=4