Fallback to file based URI when document context URI is unset

For files not synced after the previous release, context uri is unset.
This results in failure to save chat messages that retrieve documents
as the uri field cannot be unset so pre save validation fails

src/khoj/database/models/__init__.py

@@ -14,7 +14,7 @@ from django.dispatch import receiver
 from pgvector.django import VectorField
 from phonenumber_field.modelfields import PhoneNumberField
 from pydantic import BaseModel as PydanticBaseModel
-from pydantic import Field
+from pydantic import Field, model_validator
 
 logger = logging.getLogger(__name__)
 
@@ -26,6 +26,19 @@ class Context(PydanticBaseModel):
     uri: str
     query: Optional[str] = None
 
+    @model_validator(mode="after")
+    def set_uri_fallback(self):
+        """Set the URI to existing deeplink URI. Fallback to file based URI if unset."""
+        if self.uri and self.uri.strip():
+            self.uri = self.uri
+        elif self.file and (self.file.startswith("http") or self.file.startswith("file://")):
+            self.uri = self.file
+        elif self.file:
+            self.uri = f"file://{self.file}"
+        else:
+            self.uri = None
+        return self
+
 
 class CodeContextFile(PydanticBaseModel):
     filename: str