fix(enrich): set Temperature=0.1 on enrich_b and similarity call sites

Deterministic output is preferable for extraction and classification
tasks. Temperature=0.1 also enables the if-gate in gemini.go that
forwards the value to the Gemini API config.

Also add llm_enricher.go (renamed from mistral.go) with the temperature
field applied.

backend/internal/domain/discovery/enrich/llm_enricher.go

@@ -91,6 +91,7 @@ func (e *ProviderLLMEnricher) EnrichMissing(ctx context.Context, req LLMRequest)
 		JSONMode:     true,
 		Grounded:     true,
 		CallType:     "enrich_b",
+		Temperature:  0.1,
 	})
 	if err != nil {
 		return Enrichment{}, fmt.Errorf("chat: %w", err)

backend/internal/domain/discovery/enrich/similarity.go

@@ -119,6 +119,7 @@ func (c *SimilarityClassifierLLM) Classify(ctx context.Context, a, b SimilarityR
 		JSONMode:     true,
 		Grounded:     false,
 		CallType:     "similarity",
+		Temperature:  0.1,
 	})
 	if err != nil {
 		return Verdict{}, fmt.Errorf("chat: %w", err)