Return 500 errors for missing email templates and AI prompts

Changed configuration error handling to throw InvalidOperationException instead of silently using fallback values. This ensures:

1. Missing email templates (critical config) → 500 error to UI
2. Missing AI prompts (critical config) → 500 error to UI
3. Clear error messages indicating config issue
4. Prompts administrators to check database seeding

Services updated:
- EmailTemplateService.Get() throws for missing template
- CvMatcherService.ScorePairAsync() throws for missing AI prompt

This prevents silent failures with degraded service quality and makes it obvious to users that the system has a configuration problem that needs fixing.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Apis/email-data/Services/EmailTemplateService.cs

@@ -37,8 +37,9 @@ public sealed class EmailTemplateService : IEmailTemplateService
             && _valueCache.TryGetValue(CacheKey(key, "en"), out var fallback))
             return fallback;
 
-        _logger.LogWarning("Email template not found: key={Key}, language={Language}", key, language);
-        return key;
+        throw new InvalidOperationException(
+            $"Email template not found: key='{key}', language='{language}'. " +
+            $"This is a configuration error. Ensure the email.Templates table is properly seeded.");
     }
 
     /// <inheritdoc />