refactor(pylon): group backend-specific settings into per-backend config

Description

Pylon selects its upstream engine dialect with one flag, --pylon-upstream-backend <dynamo|passthrough>. Backend-specific settings are flat flags today; the priority ceiling (--pylon-priority-ceiling) applies to the Dynamo mapping but is not grouped with the backend choice.

When a second real backend lands, flat flags stop scaling: each backend brings its own settings, and unrelated flags would apply to backends that ignore them. Introduce a per-backend config block (one section per backend, only the active one read) so settings live with the dialect they configure.

Definition of Done

• Backend-specific settings are grouped per backend; a setting for one backend cannot silently apply to another.
• The existing flags keep working or have a documented migration.
• docs/api-gateway-contract.md reflects the config surface.

Resources

• Backend enum and Dynamo module: src/libraries/rust/stargate/crates/pylon-lib/src/quic_http_tunnel/backend.rs
• Flags: src/libraries/rust/stargate/crates/pylon/src/main.rs