fix(usage): centralize cache-inclusive app set and cover grokbuild in cost backfill

The cost backfill hardcoded codex|gemini as cache-inclusive apps, so
grokbuild TOTAL-semantics rows were priced on full input tokens with
cache reads double-counted. Converge the writer (proxy logger and
calculator) and the backfill recompute onto a single
sql_helpers::is_cache_inclusive_app predicate backed by the existing
CACHE_INCLUSIVE_APP_TYPES constant, and add a regression test for the
grokbuild backfill path.

src-tauri/src/proxy/usage/calculator.rs

@@ -59,7 +59,8 @@ impl CostCalculator {
         pricing: &ModelPricing,
         cost_multiplier: Decimal,
     ) -> CostBreakdown {
-        let input_includes_cache_read = matches!(app_type, "codex" | "gemini" | "grokbuild");
+        let input_includes_cache_read =
+            crate::services::sql_helpers::is_cache_inclusive_app(app_type);
         Self::calculate_with_cache_semantics(
             usage,
             pricing,

src-tauri/src/proxy/usage/logger.rs

@@ -122,7 +122,7 @@ impl<'a> UsageLogger<'a> {
 
         let created_at = chrono::Utc::now().timestamp();
         let input_token_semantics =
-            if matches!(log.app_type.as_str(), "codex" | "gemini" | "grokbuild") {
+            if crate::services::sql_helpers::is_cache_inclusive_app(log.app_type.as_str()) {
                 INPUT_TOKEN_SEMANTICS_TOTAL
             } else {
                 INPUT_TOKEN_SEMANTICS_FRESH