feat(ai-gateway): add project usage governance

crates/temps-ai-gateway/Cargo.toml

@@ -40,3 +40,4 @@ tower = { workspace = true }
 http-body-util = { workspace = true }
 http = { workspace = true }
 hyper = { workspace = true }
+temps-database = { path = "../temps-database" }

crates/temps-ai-gateway/src/error.rs

@@ -14,6 +14,48 @@ pub enum AiGatewayError {
     #[error("Model '{model}' is not allowed in scope '{scope}'")]
     ModelNotAllowed { model: String, scope: String },
 
+    #[error(
+        "AI gateway rate limit exceeded for scope '{scope}': maximum {limit_per_minute} requests per minute; retry in {retry_after_seconds}s"
+    )]
+    RateLimitExceeded {
+        scope: String,
+        limit_per_minute: i64,
+        retry_after_seconds: u64,
+    },
+
+    #[error(
+        "AI gateway monthly budget exceeded for scope '{scope}': spent {spent_microcents} of {limit_microcents} microcents"
+    )]
+    MonthlyBudgetExceeded {
+        scope: String,
+        spent_microcents: i64,
+        limit_microcents: i64,
+    },
+
+    #[error("No pricing is configured for model '{model}' required by budget scope '{scope}'")]
+    PricingUnavailable { model: String, scope: String },
+
+    #[error(
+        "AI gateway budget scope '{scope}' requires max_tokens so spend can be reserved safely"
+    )]
+    BudgetRequiresMaxTokens { scope: String },
+
+    #[error("AI gateway budget scope '{scope}' cannot safely project remote image token cost")]
+    BudgetProjectionUnavailable { scope: String },
+
+    #[error("Invalid AI gateway governance config for scope '{scope}': {field}={value} must be non-negative")]
+    InvalidGovernanceConfig {
+        scope: String,
+        field: &'static str,
+        value: i64,
+    },
+
+    #[error("Invalid AI gateway governance scope '{scope}'; expected instance, project:<id>, environment:<id>, or token:<id>")]
+    InvalidGovernanceScope { scope: String },
+
+    #[error("AI gateway governance config for scope '{scope}' was not found")]
+    GovernanceConfigNotFound { scope: String },
+
     #[error("Upstream provider error for model '{model}': {status} {message}")]
     UpstreamError {
         model: String,