fix: define DEFAULT_NEW_USER_MAX_COST_USD and repair test AppConfig initializers

Pre-existing breakage on this branch (introduced by the billing commits),
unrelated to the Gemini work but blocking both `cargo build` and
`cargo test`:

- admin/users.rs imported routes::auth::DEFAULT_NEW_USER_MAX_COST_USD,
  which was never defined. Add it to auth.rs as $1.0 (the billing design's
  "new users default to $1" starter allowance).
- AppConfig gained `payment` and `pixelle_video` fields but several test
  helpers still used struct literals missing them. Initialize both with
  Default::default() at each affected test site.

src/routes/comfyui/mod.rs

@@ -127,6 +127,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: ComfyUIConfig::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {