fix: OCR escalation falls back to tier-1 result when OCR can't run

OCR is an enhancement, not a gate. Previously, escalating a scanned doc to the
OCR tier returned the OCR result unconditionally -- so with DOCUMENT_OCR_ENABLED
=true but no backend configured (no gateway URL / no MISTRAL_API_KEY) the OCR
processor returned success=False and the whole document was marked failed and
skipped: strictly worse than leaving OCR off (where it would at least index the
tier-1 text).

Now the registry keeps the tier-1 fast result when the OCR escalation doesn't
succeed (no backend, API down, empty output), logging a warning. A
misconfiguration degrades gracefully instead of dropping scanned docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/document_processors/registry.py

@@ -264,7 +264,7 @@ class ProcessorRegistry:
                     filename or "<bytes>",
                     reason,
                 )
-                return await self._run_processor(
+                ocr_result = await self._run_processor(
                     ocr,
                     content,
                     content_type,
@@ -273,6 +273,19 @@ class ProcessorRegistry:
                     progress_callback,
                     escalated=True,
                 )
+                # OCR is an enhancement, not a gate: if it can't run (no backend
+                # configured / API down) or returns nothing, keep the tier-1
+                # result rather than failing the document. Otherwise an operator
+                # who sets DOCUMENT_OCR_ENABLED=true without credentials would
+                # make scanned docs fail entirely -- strictly worse than off.
+                if ocr_result.success:
+                    return ocr_result
+                logger.warning(
+                    "OCR escalation did not succeed for %s (%s); keeping the "
+                    "tier-1 result",
+                    filename or "<bytes>",
+                    ocr_result.metadata.get("parse_failed_reason", "error"),
+                )
 
         return result