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/admin/compat.rs

@@ -2840,6 +2840,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: Default::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/admin/downstream_keys.rs

@@ -412,6 +412,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: Default::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/auth.rs

@@ -19,6 +19,11 @@ use serde_json::{json, Value};
 use std::net::{IpAddr, SocketAddr};
 use tracing::{info, warn};
 
+/// Default spending cap (USD) granted to a newly created user's auto-generated
+/// API key. Per the billing design (新用户默认 $1 额度), new users start with a
+/// $1 allowance on their default downstream key.
+pub const DEFAULT_NEW_USER_MAX_COST_USD: f64 = 1.0;
+
 // ── Request / Response types ─────────────────────────────────────────
 
 #[derive(Debug, Deserialize)]
@@ -1019,6 +1024,7 @@ mod tests {
                 },
                 comfyui: Default::default(),
                 payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

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 {

src/routes/comfyui_agent.rs

@@ -1305,6 +1305,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: Default::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/events.rs

@@ -536,6 +536,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: Default::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/money_printer.rs

@@ -2071,6 +2071,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui,
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/oauth.rs

@@ -2550,6 +2550,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: ComfyUIConfig::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {

src/routes/video_agent_legacy.rs

@@ -696,6 +696,8 @@ mod tests {
                     template_proxy_url: None,
                 },
                 comfyui: ComfyUIConfig::default(),
+                payment: Default::default(),
+                pixelle_video: Default::default(),
             },
             pool,
             BuiltinKeys {