feat: add FOREMAN_KEEP_ALIVE config for worker model residency

Allow configuring how long the worker model stays resident on the Ollama
target after a request via FOREMAN_KEEP_ALIVE env var. Accepts Ollama
duration strings ("-1" forever, "0" unload, "15m", "1h", etc). Defaults
to "-1" (pin forever). The embedder warm-up is unaffected and always
uses keep_alive=-1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/config/config_test.go

@@ -31,6 +31,9 @@ func TestLoad_Defaults(t *testing.T) {
 	if cfg.PollInterval != 30*time.Second {
 		t.Errorf("PollInterval = %v, want %v", cfg.PollInterval, 30*time.Second)
 	}
+	if cfg.KeepAlive != "-1" {
+		t.Errorf("KeepAlive = %q, want %q", cfg.KeepAlive, "-1")
+	}
 }
 
 func TestLoad_AllEnvVars(t *testing.T) {
@@ -42,6 +45,7 @@ func TestLoad_AllEnvVars(t *testing.T) {
 	t.Setenv("FOREMAN_DB_PATH", "/data/foreman.db")
 	t.Setenv("FOREMAN_POLL_INTERVAL", "1m")
 	t.Setenv("FOREMAN_WEBHOOK_SECRET", "hmac-key")
+	t.Setenv("FOREMAN_KEEP_ALIVE", "15m")
 
 	cfg, err := Load()
 	if err != nil {
@@ -72,6 +76,9 @@ func TestLoad_AllEnvVars(t *testing.T) {
 	if cfg.WebhookSecret != "hmac-key" {
 		t.Errorf("WebhookSecret = %q", cfg.WebhookSecret)
 	}
+	if cfg.KeepAlive != "15m" {
+		t.Errorf("KeepAlive = %q, want %q", cfg.KeepAlive, "15m")
+	}
 }
 
 func TestLoad_MissingOllamaURL(t *testing.T) {