fix(db-models): fixed sqlalchemy warnings in DB models.

rhodecode/model/db.py

@@ -1779,13 +1779,13 b' class Repository(Base, BaseModel):'
         'PullRequest',
         primaryjoin='PullRequest.source_repo_id==Repository.repo_id',
         cascade="all, delete-orphan",
-        #back_populates="pr_source"
+        overlaps="source_repo"
     )
     pull_requests_target = relationship(
         'PullRequest',
         primaryjoin='PullRequest.target_repo_id==Repository.repo_id',
         cascade="all, delete-orphan",
-        #back_populates="pr_target"
+        overlaps="target_repo"
     )
 
     ui = relationship('RepoRhodeCodeUi', cascade="all")
@@ -4303,7 +4303,7 b' class _PullRequestBase(BaseModel):'
         return relationship(
             'Repository',
             primaryjoin=f'{cls.__name__}.source_repo_id==Repository.repo_id',
-            #back_populates=''
+            overlaps="pr_source"
         )
 
     @property
@@ -4314,7 +4314,8 b' class _PullRequestBase(BaseModel):'
     def target_repo(cls):
         return relationship(
             'Repository',
-            primaryjoin=f'{cls.__name__}.target_repo_id==Repository.repo_id'
+            primaryjoin=f'{cls.__name__}.target_repo_id==Repository.repo_id',
+            overlaps="pr_target"
         )
 
     @property