feat(backend): Ollama embeddings preflight on session create/resume (503 hard-fail)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/config.ts

@@ -4,6 +4,7 @@ export interface AppConfig {
   defaults: { provider?: string; model?: string; thinking?: string };
   maxPiProcesses: number;
   settingsFile: string;
+  ollamaEnsureTimeoutMs: number;
 }
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
   return {
@@ -15,5 +16,6 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
     maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
     settingsFile: env.SETTINGS_FILE ?? "data/settings.json",
+    ollamaEnsureTimeoutMs: Number(env.OLLAMA_ENSURE_TIMEOUT_MS ?? 60000),
   };
 }