Persist processing-log retention settings and wire cleanup defaults

backend/app/schemas/settings.py

@@ -127,6 +127,20 @@ class DisplaySettingsUpdateRequest(BaseModel):
     log_typing_animation_enabled: bool | None = None
 
 
+class ProcessingLogRetentionSettingsResponse(BaseModel):
+    """Represents retention limits used when pruning processing pipeline logs."""
+
+    keep_document_sessions: int = Field(default=2, ge=0, le=20)
+    keep_unbound_entries: int = Field(default=80, ge=0, le=400)
+
+
+class ProcessingLogRetentionSettingsUpdateRequest(BaseModel):
+    """Represents partial updates for processing log retention limits."""
+
+    keep_document_sessions: int | None = Field(default=None, ge=0, le=20)
+    keep_unbound_entries: int | None = Field(default=None, ge=0, le=400)
+
+
 class PredefinedPathEntryResponse(BaseModel):
     """Represents one predefined logical path with global discoverability scope."""
 
@@ -200,6 +214,7 @@ class AppSettingsResponse(BaseModel):
 
     upload_defaults: UploadDefaultsResponse
     display: DisplaySettingsResponse
+    processing_log_retention: ProcessingLogRetentionSettingsResponse
     handwriting_style_clustering: HandwritingStyleSettingsResponse
     predefined_paths: list[PredefinedPathEntryResponse] = Field(default_factory=list)
     predefined_tags: list[PredefinedTagEntryResponse] = Field(default_factory=list)
@@ -212,6 +227,7 @@ class AppSettingsUpdateRequest(BaseModel):
 
     upload_defaults: UploadDefaultsUpdateRequest | None = None
     display: DisplaySettingsUpdateRequest | None = None
+    processing_log_retention: ProcessingLogRetentionSettingsUpdateRequest | None = None
     handwriting_style_clustering: HandwritingStyleSettingsUpdateRequest | None = None
     predefined_paths: list[PredefinedPathEntryUpdateRequest] | None = None
     predefined_tags: list[PredefinedTagEntryUpdateRequest] | None = None