feat(config): add OnboardSummarizerModel + OnboardMaxVideoSeconds (ADR-028)

Two knobs for the onboarding-burst quality work:
- TAPIR_ONBOARD_SUMMARIZER_MODEL (default iguana/gemma4-26b): the stronger model
  the burst leads its chain with; empty collapses the burst onto the shared
  processor (lookupOr, so explicit-empty disables).
- TAPIR_ONBOARD_MAX_VIDEO_SECONDS (default 14400/4h): upper duration bound for
  burst picks; 0 disables, negative clamps to 0.

Table-driven tests cover defaults, explicit, disable, and invalid input.

internal/config/config_test.go

@@ -214,3 +214,62 @@ func TestValidateForAuth_PassesWhenComplete(t *testing.T) {
 		t.Errorf("ValidateForAuth: unexpected error %v", err)
 	}
 }
+
+func TestLoad_OnboardSummarizerModel(t *testing.T) {
+	cases := []struct {
+		name, env string
+		set       bool
+		want      string
+	}{
+		{"default", "", false, defaultOnboardSummarizerModel},
+		{"explicit", "koala/some-model", true, "koala/some-model"},
+		{"empty disables (collapses to shared processor)", "", true, ""},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			env := map[string]string{}
+			if c.set {
+				env["TAPIR_ONBOARD_SUMMARIZER_MODEL"] = c.env
+			}
+			setEnv(t, env)
+			cfg, err := Load()
+			if err != nil {
+				t.Fatalf("Load: %v", err)
+			}
+			if cfg.OnboardSummarizerModel != c.want {
+				t.Fatalf("OnboardSummarizerModel = %q, want %q", cfg.OnboardSummarizerModel, c.want)
+			}
+		})
+	}
+}
+
+func TestLoad_OnboardMaxVideoSeconds(t *testing.T) {
+	cases := []struct {
+		name, env string
+		want      int
+	}{
+		{"default", "", defaultOnboardMaxVideoSeconds},
+		{"explicit", "7200", 7200},
+		{"zero disables", "0", 0},
+		{"negative clamps to zero", "-9", 0},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			setEnv(t, map[string]string{"TAPIR_ONBOARD_MAX_VIDEO_SECONDS": c.env})
+			cfg, err := Load()
+			if err != nil {
+				t.Fatalf("Load: %v", err)
+			}
+			if cfg.OnboardMaxVideoSeconds != c.want {
+				t.Fatalf("OnboardMaxVideoSeconds = %d, want %d", cfg.OnboardMaxVideoSeconds, c.want)
+			}
+		})
+	}
+}
+
+func TestLoad_OnboardMaxVideoSecondsInvalid(t *testing.T) {
+	setEnv(t, map[string]string{"TAPIR_ONBOARD_MAX_VIDEO_SECONDS": "long"})
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: want error for non-numeric TAPIR_ONBOARD_MAX_VIDEO_SECONDS")
+	}
+}