Fix MCP crash when DB key exists without NEXUS_MASTER_KEY

KeyResolver now wraps getApiKey in try/catch and falls back to env instead of
crashing startup. ProvidersStore.getApiKey returns undefined when the cipher is
disabled rather than throwing. Server starts cleanly with env-only, and uses the
encrypted DB key when the master key is present.

NexusAI/packages/core/src/db/providers-store.ts

@@ -133,12 +133,17 @@ export class ProvidersStore {
     this.db.prepare('DELETE FROM providers WHERE name = ?').run(name);
   }
 
-  /** Return the decrypted API key for a provider, or undefined. */
+  /**
+   * Return the decrypted API key for a provider, or undefined.
+   * Returns undefined (instead of throwing) when no master key is configured,
+   * so callers can gracefully fall back to env keys.
+   */
   getApiKey(name: string): string | undefined {
     const row = this.db.prepare('SELECT api_key_enc FROM providers WHERE name = ?').get(name) as
       | { api_key_enc: string | null }
       | undefined;
     if (!row?.api_key_enc) return undefined;
+    if (!this.cipher.enabled) return undefined;
     return this.cipher.decrypt(row.api_key_enc);
   }
 

NexusAI/packages/core/src/security/resolver.ts

@@ -45,7 +45,18 @@ export class KeyResolver {
   resolve(providerName: string, defaultBaseUrl: string): ResolvedProvider {
     const dbRow = this.store?.get(providerName);
     const envKey = this.envKeyFor(providerName);
-    const dbKey = this.store?.getApiKey(providerName);
+    // DB key may be unreadable (no master key / rotated key). Degrade gracefully:
+    // fall back to env instead of crashing startup.
+    let dbKey: string | undefined;
+    try {
+      dbKey = this.store?.getApiKey(providerName);
+    } catch (err) {
+      log.warn(
+        `could not read stored key for "${providerName}" (falling back to env)`,
+        err instanceof Error ? err.message : String(err)
+      );
+      dbKey = undefined;
+    }
 
     const apiKey = envKey ?? dbKey;
     const keySource: ResolvedProvider['keySource'] = envKey ? 'env' : dbKey ? 'db' : 'none';