feat(summarizer): move local fallback off koala to iguana/gemma4-26b

koala now carries other GPU loads, so the first fallback should not run there.
Change the default chain to koala/phi4-mini → iguana/gemma4-26b → berget/mistral-small:
the local fallback now runs on iguana (M2 Ultra headroom, different host = different
egress IP for the rare fallback fetch). gemma4-26b is the brain-validated homelab
general-purpose model (agentsquad H2/H3 executor) and returned valid summary JSON on
the real prompt in a smoke test (~37s incl. cold-load — fine for a fallback path).

Pure config default (TAPIR_FALLBACK_MODEL); chain mechanism (ADR-022) unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/config/config.go

@@ -33,7 +33,10 @@ type Config struct {
 	SummarizerModel string
 	// FallbackModel is the LOCAL fallback alias tried when the primary fails or
 	// returns unparseable output (ADR-022). Kept local so content stays on the
-	// homelab stack. Empty disables it. Default a bigger-context local model.
+	// homelab stack. Default is an IGUANA model (not koala) so the fallback runs
+	// on a different host than the koala primary — koala carries other loads, and
+	// a different host also means a different egress IP for the (rare) fallback.
+	// Empty disables it.
 	FallbackModel string
 	// CloudFallbackModel is the worst-case EXTERNAL fallback alias, tried only
 	// after every local endpoint has failed (ADR-022). For client deployments set
@@ -148,7 +151,7 @@ func (c Config) DexConfigured() bool { return strings.TrimSpace(c.OIDCIssuer) !=
 const (
 	defaultGatewayURL            = "http://koala:30401/v1"
 	defaultSummarizerModel       = "koala/phi4-mini"
-	defaultFallbackModel         = "koala/phi4-14b"
+	defaultFallbackModel         = "iguana/gemma4-26b"
 	defaultCloudFallbackModel    = "berget/mistral-small"
 	defaultSummaryMaxTokens      = 1500
 	defaultMaxTranscriptChars    = 18000