fix: guard session['auth_source_manager'] access in BatchProcess.start

processes.py:322 unconditionally indexed session['auth_source_manager']
to gate the KRB5CCNAME copy. When the Flask session does not have the
auth_source_manager key (test request contexts that bypass the login
flow, or sessions whose login hasn't populated this key yet), the
indexing raises KeyError, aborting BatchProcess.start() before it ever
spawns the subprocess. The error has no obvious traceback in the
runtests harness because it bubbles up through BackupCreateJob's mock
infrastructure - the test just records ERROR with no message.

Replace [...] with .get(...) chains so a missing auth_source_manager
yields None and the Kerberos branch is skipped, matching the original
intent.

Surfaced by BatchProcessTest "When backup server" / "When backup
globals" scenarios which mock current_user but leave session in its
default unauthenticated state. Same shape on master; the fix is a
one-line robustness improvement.