fix: bump shuttle to 0.51.0

[jonaro00]

• Bumps shuttle deps to latest version
• Updates Shuttle.toml configuration for shuttle.dev platform

Solves shuttle-hq/shuttle#1937

Tested by doing loco new, cargo loco generate deployment (shuttle), applying the changes in this PR by hand, copying boilerplate from development.yaml to production.yaml, then shuttle deploy. shuttle run also works as expected.

[jonaro00]

Another optional improvement would be to add more static asset paths to the Shuttle.toml file, such as frontend/dist. This is template dependant, so it could be done preemptively or based on which template is chosen.