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/cv-matcher-api/Services/CvMatcherService.cs

@@ -125,9 +125,9 @@ public sealed class CvMatcherService : ICvMatcherService
         var evidence = evidenceChunks.Count > 0 ? string.Join("\n\n", evidenceChunks.Take(4)) : Limit(job.Text, 4000);
 
         var systemPrompt = await _aiPrompts.GetAsync("ai.cv-match.system-prompt", language, ct)
-            ?? (language == "ro"
-                ? "Ești un motor strict de potrivire CV-job. Returnează doar JSON. Punctează realist între 0 și 100."
-                : "You are a strict CV-to-job matching engine. Return JSON only. Score realistically from 0 to 100.");
+            ?? throw new InvalidOperationException(
+                $"AI prompt not found: key='ai.cv-match.system-prompt', language='{language}'. " +
+                $"This is a configuration error. Ensure the cvMatcher.AiPrompts table is properly seeded with language-specific prompts.");
 
         var userPrompt = $"""
             CV:

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 />