fix(review): _enum_fields validation, gate classify_from_text flags, OCR warnings

Address PR #858 review round 3:

- document_tier1_engine / document_ocr_provider now validate + normalize via
  Settings.__post_init__ _enum_fields (the repo's canonical opt-in-enum pattern;
  case-insensitive) instead of dynaconf Validators. A typo now raises ValueError
  at load and "Gateway" normalizes to "gateway".
- classify_from_text gates no_text_layer/bad_text_layer on ocr_frac >=
  OCR_PAGE_FRACTION, matching classify_pdf -- a "fast"-routed doc with a few junk
  pages no longer emits a misleading flag (keeps the shadow vs hot-path
  classification metrics consistent).
- build_ocr_backend warns when an EXPLICIT provider is misconfigured
  (gateway without EMBEDDING_GATEWAY_URL, mistral without MISTRAL_API_KEY)
  instead of silently returning None.
- Pypdfium2FastProcessor.health_check probes the import; documented why
  OcrProcessor.health_check is unconditionally True (lazy per-tenant backends).
- Removed the leftover per-boundary / per-chunk debug logging loops.

Tests: enum normalization + rejection for the two new settings.

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

nextcloud_mcp_server/vector/processor.py

@@ -595,14 +595,6 @@ async def _index_document(
                         len(page_boundaries),
                         len(content),
                     )
-                    # Log first 3 page boundaries for debugging
-                    for boundary in page_boundaries[:3]:
-                        logger.debug(
-                            "  Page %s: offsets [%s:%s]",
-                            boundary["page"],
-                            boundary["start_offset"],
-                            boundary["end_offset"],
-                        )
                     # Verify last boundary matches text length
                     if page_boundaries:
                         last_boundary = page_boundaries[-1]
@@ -659,16 +651,6 @@ async def _index_document(
                 file_path,
             )
 
-            # Log first 3 chunks to see their page assignments
-            for i, chunk in enumerate(chunks[:3]):
-                logger.debug(
-                    "  Chunk %s: page=%s, offsets=[%s:%s]",
-                    i,
-                    chunk.page_number,
-                    chunk.start_offset,
-                    chunk.end_offset,
-                )
-
             # Warning if NO page numbers were assigned
             if assigned_count == 0:
                 logger.warning(