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_test.go

@@ -69,3 +69,78 @@ func TestRouter_NilFallback(t *testing.T) {
 	_, err := r.Complete(context.Background(), "sys", "user")
 	assert.Error(t, err)
 }
+
+func TestRouter_FallsBackToTertiaryWhenPrimaryAndFallbackFail(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "unavailable", http.StatusServiceUnavailable)
+	}))
+	defer fail.Close()
+	tertiary := mockServer(t, "from-tertiary")
+	defer tertiary.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fail.URL, "", "m", time.Second),
+		Tertiary: New(tertiary.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	require.NoError(t, err)
+	assert.Equal(t, "from-tertiary", out)
+}
+
+func TestRouter_TertiaryNotCalledWhenFallbackSucceeds(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "unavailable", http.StatusServiceUnavailable)
+	}))
+	defer fail.Close()
+	fallback := mockServer(t, "from-fallback")
+	defer fallback.Close()
+	tertiary := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Error("tertiary must not be called when fallback succeeds")
+	}))
+	defer tertiary.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fallback.URL, "", "m", time.Second),
+		Tertiary: New(tertiary.URL, "", "m", time.Second),
+	}
+	out, err := r.Complete(context.Background(), "sys", "user")
+	require.NoError(t, err)
+	assert.Equal(t, "from-fallback", out)
+}
+
+func TestRouter_AllThreeFail(t *testing.T) {
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fail.URL, "", "m", time.Second),
+		Tertiary: New(fail.URL, "", "m", time.Second),
+	}
+	_, err := r.Complete(context.Background(), "sys", "user")
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "primary llm")
+	assert.Contains(t, err.Error(), "fallback llm")
+	assert.Contains(t, err.Error(), "tertiary llm")
+}
+
+func TestRouter_NilTertiary(t *testing.T) {
+	// Existing 2-tier deployments (no BRAIN_LLM_TERTIARY_URL) must keep working
+	// exactly as before — Tertiary is opt-in, not required.
+	fail := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "err", http.StatusBadGateway)
+	}))
+	defer fail.Close()
+
+	r := &Router{
+		Primary:  New(fail.URL, "", "m", time.Second),
+		Fallback: New(fail.URL, "", "m", time.Second),
+	}
+	_, err := r.Complete(context.Background(), "sys", "user")
+	assert.Error(t, err)
+	assert.NotContains(t, err.Error(), "tertiary")
+}