The bare `except Exception` in HTTPSource.discover() silently
swallowed all errors from HEAD requests — including configuration
errors (bad auth, invalid headers) and programming errors (TypeError,
AttributeError) — treating them identically to network failures by
emitting UPSERT with no revision.
Narrow the catch to httpx.TransportError (the umbrella for
ConnectError, TimeoutException, etc.) and add a debug log. Other
exceptions now propagate to the poller's circuit breaker where they
surface as failures instead of being silently retried forever.
