fix: invalid session_max_age_secs is silently ignored, causing wrong session lifetime #15366

Description

max_age() catches ValueError when SESSION_MAX_AGE_SECS is malformed, logs it, and silently falls back to 30 days. This hides configuration mistakes and can materially change authentication behavior (sessions lasting far longer or shorter than intended) without failing fast.

Severity: medium
File: gear/gear/auth_utils.py

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.