feat(brain): add Gemini as brain_answer's third LLM tier

llm.Router was primary-then-fallback (berget -> iguana ollama). Both
have failed together at least once in production (berget 5xx outage +
a cold fallback model exceeding the shared timeout budget), leaving
brain_answer/brain_classify fully down.

Router gains an optional Tertiary *Client, tried only if both Primary
and Fallback fail. Nil-safe — existing 2-tier deployments (no
BRAIN_LLM_TERTIARY_URL) behave exactly as before.

main.go wires it from BRAIN_LLM_TERTIARY_URL/MODEL + GEMINI_API_KEY,
pointed at Google's OpenAI-compatible Gemini endpoint
(generativelanguage.googleapis.com/v1beta/openai/) — llm.Client already
speaks OpenAI-shaped chat-completions, so no new client type needed.

Tests: TestRouter_FallsBackToTertiaryWhenPrimaryAndFallbackFail,
TestRouter_TertiaryNotCalledWhenFallbackSucceeds, TestRouter_AllThreeFail,
TestRouter_NilTertiary (2-tier backward-compat). Full suite green
(both modules, -race), golangci-lint clean.

Deployment env vars (infra repo, separate commit — GitOps, TBD there
too): BRAIN_LLM_TERTIARY_URL, BRAIN_LLM_TERTIARY_MODEL, GEMINI_API_KEY
secretRef.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/llm/router.go

@@ -5,14 +5,18 @@ import (
 	"fmt"
 )
 
-// Router calls Primary first; on any error falls back to Fallback.
-// Fallback may be nil, in which case primary errors are returned directly.
+// Router calls Primary first; on any error falls back to Fallback, then to
+// Tertiary. Fallback and Tertiary may each be nil — a nil tier is skipped and
+// the chain returns the errors accumulated so far.
 type Router struct {
 	Primary  *Client
 	Fallback *Client
+	Tertiary *Client
 }
 
-// Complete implements pipeline.CompleteFunc, routing through Primary then Fallback.
+// Complete implements pipeline.CompleteFunc, routing through Primary, then
+// Fallback, then Tertiary — trying each configured tier in order and
+// returning the first success.
 func (r *Router) Complete(ctx context.Context, system, user string) (string, error) {
 	out, err := r.Primary.Complete(ctx, system, user)
 	if err == nil {
@@ -22,8 +26,15 @@ func (r *Router) Complete(ctx context.Context, system, user string) (string, err
 		return "", fmt.Errorf("primary llm: %w", err)
 	}
 	out, err2 := r.Fallback.Complete(ctx, system, user)
-	if err2 != nil {
+	if err2 == nil {
+		return out, nil
+	}
+	if r.Tertiary == nil {
 		return "", fmt.Errorf("primary llm: %w; fallback llm: %v", err, err2)
 	}
+	out, err3 := r.Tertiary.Complete(ctx, system, user)
+	if err3 != nil {
+		return "", fmt.Errorf("primary llm: %w; fallback llm: %v; tertiary llm: %v", err, err2, err3)
+	}
 	return out, nil
 }