Add MinerU document ingestion support

Normalize uploaded documents into structured markdown, add clearer parser
errors for missing dependencies, and cover the ingestion flow with
backend tests. This also replaces deprecated UTC timestamp helpers in
the touched backend paths so the knowledge pipeline stays warning-free.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/schemas/document.py

@@ -11,6 +11,13 @@ class DocumentOut(BaseModel):
     summary: str | None
     chunk_count: int
     is_indexed: bool
+    ingestion_status: str
+    ingestion_error: str | None
+    indexed_at: datetime | None
+    parser_version: str | None
+    index_version: str | None
+    normalized_format: str | None
+    folder_id: str | None
     created_at: datetime
 
     model_config = {"from_attributes": True}
@@ -25,6 +32,10 @@ class DocumentChunkOut(BaseModel):
     model_config = {"from_attributes": True}
 
 
+class DocumentChunkUpdate(BaseModel):
+    content: str
+
+
 class SearchRequest(BaseModel):
     query: str
     top_k: int = 5