housekeep(fastapi): rework aurweb.db session API

Changes:
-------
- Add aurweb.db.get_session()
    - Returns aurweb.db's global `session` instance
    - Provides us a way to change the implementation of the session
      instance without interrupting user code.
- Use aurweb.db.get_session() in session API methods
- Add docstrings to session API methods
- Refactor aurweb.db.delete
    - Normalize aurweb.db.delete to an alias of session.delete
- Refresh instances in places we depend on their non-PK columns
  being up to date.

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/testing/__init__.py

@@ -19,7 +19,7 @@ def references_graph(table):
         "regexp_1": r'(?i)\s+references\s+("|\')?',
         "regexp_2": r'("|\')?\s*\(',
     }
-    cursor = aurweb.db.session.execute(query, params=params)
+    cursor = aurweb.db.get_session().execute(query, params=params)
     return [row[0] for row in cursor.fetchall()]
 
 
@@ -51,7 +51,7 @@ def setup_test_db(*args):
     db_backend = aurweb.config.get("database", "backend")
 
     if db_backend != "sqlite":  # pragma: no cover
-        aurweb.db.session.execute("SET FOREIGN_KEY_CHECKS = 0")
+        aurweb.db.get_session().execute("SET FOREIGN_KEY_CHECKS = 0")
     else:
         # We're using sqlite, setup tables to be deleted without violating
         # foreign key constraints by graphing references.
@@ -59,10 +59,10 @@ def setup_test_db(*args):
             references_graph(table) for table in tables))
 
     for table in tables:
-        aurweb.db.session.execute(f"DELETE FROM {table}")
+        aurweb.db.get_session().execute(f"DELETE FROM {table}")
 
     if db_backend != "sqlite":  # pragma: no cover
-        aurweb.db.session.execute("SET FOREIGN_KEY_CHECKS = 1")
+        aurweb.db.get_session().execute("SET FOREIGN_KEY_CHECKS = 1")
 
     # Expunge all objects from SQLAlchemy's IdentityMap.
-    aurweb.db.session.expunge_all()
+    aurweb.db.get_session().expunge_all()