Add universal document decoder with multi-format support (#705)

Add universal document decoder with multi-format support
using 'unstructured'.

New universal decoder service powered by the unstructured
library, handling DOCX, XLSX, PPTX, HTML, Markdown, CSV, RTF,
ODT, EPUB and more through a single service. Tables are preserved
as HTML markup for better downstream extraction. Images are
stored in the librarian but excluded from the text
pipeline. Configurable section grouping strategies
(whole-document, heading, element-type, count, size) for non-page
formats. Page-based formats (PDF, PPTX, XLSX) are automatically
grouped by page.

All four decoders (PDF, Mistral OCR, Tesseract OCR, universal)
now share the "document-decoder" ident so they are
interchangeable.  PDF-only decoders fetch document metadata to
check MIME type and gracefully skip unsupported formats.

Librarian changes: removed MIME type whitelist validation so any
document format can be ingested. Simplified routing so text/plain
goes to text-load and everything else goes to document-load.
Removed dual inline/streaming data paths — documents always use
document_id for content retrieval.

New provenance entity types (tg:Section, tg:Image) and metadata
predicates (tg:elementTypes, tg:tableCount, tg:imageCount) for
richer explainability.

Universal decoder is in its own package (trustgraph-unstructured)
and container image (trustgraph-unstructured).

trustgraph-ocr/trustgraph/decoding/ocr/pdf_decoder.py

@@ -33,7 +33,7 @@ COMPONENT_VERSION = "1.0.0"
 # Module logger
 logger = logging.getLogger(__name__)
 
-default_ident = "pdf-decoder"
+default_ident = "document-decoder"
 
 default_librarian_request_queue = librarian_request_queue
 default_librarian_response_queue = librarian_response_queue
@@ -127,6 +127,39 @@ class Processor(FlowProcessor):
         else:
             logger.warning(f"Received unexpected librarian response: {request_id}")
 
+    async def fetch_document_metadata(self, document_id, user, timeout=120):
+        """
+        Fetch document metadata from librarian via Pulsar.
+        """
+        request_id = str(uuid.uuid4())
+
+        request = LibrarianRequest(
+            operation="get-document-metadata",
+            document_id=document_id,
+            user=user,
+        )
+
+        future = asyncio.get_event_loop().create_future()
+        self.pending_requests[request_id] = future
+
+        try:
+            await self.librarian_request_producer.send(
+                request, properties={"id": request_id}
+            )
+
+            response = await asyncio.wait_for(future, timeout=timeout)
+
+            if response.error:
+                raise RuntimeError(
+                    f"Librarian error: {response.error.type}: {response.error.message}"
+                )
+
+            return response.document_metadata
+
+        except asyncio.TimeoutError:
+            self.pending_requests.pop(request_id, None)
+            raise RuntimeError(f"Timeout fetching metadata for {document_id}")
+
     async def fetch_document_content(self, document_id, user, timeout=120):
         """
         Fetch document content from librarian via Pulsar.
@@ -217,6 +250,20 @@ class Processor(FlowProcessor):
 
         logger.info(f"Decoding {v.metadata.id}...")
 
+        # Check MIME type if fetching from librarian
+        if v.document_id:
+            doc_meta = await self.fetch_document_metadata(
+                document_id=v.document_id,
+                user=v.metadata.user,
+            )
+            if doc_meta and doc_meta.kind and doc_meta.kind != "application/pdf":
+                logger.error(
+                    f"Unsupported MIME type: {doc_meta.kind}. "
+                    f"Tesseract OCR decoder only handles application/pdf. "
+                    f"Ignoring document {v.metadata.id}."
+                )
+                return
+
         # Get PDF content - fetch from librarian or use inline data
         if v.document_id:
             logger.info(f"Fetching document {v.document_id} from librarian...")