feat: harden observability config transitions and gas

[shaaibu7]

Summary

• Add structured, redacted backend logging with request and trace context.
• Centralize backend configuration validation and document env vars across services.
• Add reduced-motion-aware page transitions for primary frontend routes.
• Reduce agent registry gas estimates and document benchmark snapshots.

Testing

• Not run, per request.

Closes #335
Closes #332
Closes #329
Closes #333

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