Catching exceptions without hiding bugs

Specific enough that an unexpected failure still crashes. except Exception around an entire request handler is fine if it logs and re-raises in development; the same clause around three lines of parsing will swallow the typo that broke them.

At the boundaries: function signatures on public modules first, internals never. Run the checker in non-blocking mode for a few weeks so the noise is visible without stopping anyone, then turn on strictness one module at a time.