v2.6.51: fix PDF import — raise UI body limit, wire real analyze

- docker-compose.yml: BODY_SIZE_LIMIT=52428800 (50MB) on UI service
  — adapter-node was rejecting PDFs >512KB with 'Content-length exceeds limit'
- storage/import.go: add AnalyzeFile() public function using real PDF/EPUB parsers
- handlers_import.go: analyzeImportFile() now calls storage.AnalyzeFile() instead
  of the file-size stub, so chapter count is accurate in the preview

docker-compose.yml

@@ -307,6 +307,8 @@ services:
       # OpenTelemetry tracing
       OTEL_EXPORTER_OTLP_ENDPOINT: "${OTEL_EXPORTER_OTLP_ENDPOINT}"
       OTEL_SERVICE_NAME: "ui"
+      # Allow large PDF/EPUB uploads (adapter-node default is 512KB)
+      BODY_SIZE_LIMIT: "52428800"
       # OAuth2 providers
       GOOGLE_CLIENT_ID: "${GOOGLE_CLIENT_ID}"
       GOOGLE_CLIENT_SECRET: "${GOOGLE_CLIENT_SECRET}"