chore(saas): remove dead ErrorTrackingService island + credits path exclusion (Stirling-Tools#6718)

## What

Residual dead-code cleanup following the credits engine teardown
(Stirling-Tools#6687).

- **Delete the `ErrorTrackingService` dead island** (6 files): the
service, `UserErrorTrackerRepository`, `UserErrorTracker`,
`ProcessingErrorType`, `CreditsProperties`, and
`ErrorTrackingServiceTest`. These formed a self-referential cluster with
**zero external callers** once the credit machinery was removed.
- **Remove `/api/v1/credits/**`** from both `excludePathPatterns` blocks
in `PaygWebMvcConfig` — the credits controller no longer exists, so the
exclusion is defunct. (spotless collapsed the lists to one line.)

## Verification

- `./gradlew :saas:compileJava :saas:compileTestJava` → **BUILD
SUCCESSFUL**
- grep confirms zero dangling references to the deleted types

## Not in scope (deliberately deferred)

Destructive DB drops
(`user_credits`/`team_credits`/`user_subscription_plans` tables, dead
`payg_shadow_charge` columns, `user_error_tracker` table) are gated
behind the post-release soak (`live_ratio==1.0 ≥7d`) and tracked in a
separate bundle.

Co-authored-by: EthanHealy01 <80844253+EthanHealy01@users.noreply.github.com>

Author: ConnorYoh

app/saas/src/main/java/stirling/software/saas/config/CreditsProperties.java

@@ -62,20 +62,12 @@ public class PaygWebMvcConfig implements WebMvcConfigurer {
     public void addInterceptors(InterceptorRegistry registry) {
         registry.addInterceptor(paygChargeInterceptor)
                 .addPathPatterns("/api/**")
-                .excludePathPatterns(
-                        "/api/v1/credits/**",
-                        "/api/v1/config/**",
-                        "/api/v1/info/**",
-                        "/api/v1/admin/**")
+                .excludePathPatterns("/api/v1/config/**", "/api/v1/info/**", "/api/v1/admin/**")
                 .order(INTERCEPTOR_ORDER);
 
         registry.addInterceptor(entitlementGuard)
                 .addPathPatterns("/api/**")
-                .excludePathPatterns(
-                        "/api/v1/credits/**",
-                        "/api/v1/config/**",
-                        "/api/v1/info/**",
-                        "/api/v1/admin/**")
+                .excludePathPatterns("/api/v1/config/**", "/api/v1/info/**", "/api/v1/admin/**")
                 .order(ENTITLEMENT_GUARD_ORDER);
     }
 }