feat(api): GET /api/missions + POST /api/missions/{id}/accept

Routes call ensureSave first (mission functions assume an initialized save),
map MissionError to 404/409 and non-integer path ids to 400. OpenAPI spec in
code gains both paths + the Mission schema (12 -> 14 paths); openapi.test.ts
covers the new shapes.

lib/openapi.test.ts

@@ -8,6 +8,8 @@ describe('OpenAPI spec', () => {
       '/api/crew',
       '/api/explore',
       '/api/jump',
+      '/api/missions',
+      '/api/missions/{id}/accept',
       '/api/research',
       '/api/ship',
       '/api/ship/build',
@@ -58,6 +60,23 @@ describe('OpenAPI spec', () => {
     }
   });
 
+  it('describes the mission endpoints: GET /api/missions → Mission[], POST accept with error responses', () => {
+    const missions = openApiSpec.paths['/api/missions'].get.responses[200].content['application/json'].schema;
+    expect(missions.type).toBe('array');
+    expect((missions.items as { $ref: string }).$ref).toBe('#/components/schemas/Mission');
+
+    const props = openApiSpec.components.schemas.Mission.properties;
+    expect(Object.keys(props)).toEqual(['id', 'type', 'description', 'targetSystemId', 'conditionKind', 'progressTarget', 'progress', 'rewardKind', 'rewardQty', 'state']);
+    expect((props.type as { enum: string[] }).enum).toEqual(['DELIVER', 'EXPLORE', 'FIND']);
+    expect((props.state as { enum: string[] }).enum).toEqual(['AVAILABLE', 'ACTIVE', 'DONE']);
+
+    const post = openApiSpec.paths['/api/missions/{id}/accept'].post;
+    expect(post.responses[200].content['application/json'].schema.items.$ref).toBe('#/components/schemas/Mission');
+    for (const status of ['400', '404', '409'] as const) {
+      expect(post.responses[status].content['application/json'].schema.$ref).toBe('#/components/schemas/Error');
+    }
+  });
+
   it('describes /api/crew as an array of CrewMember and crewFound on ExploreResult', () => {
     const crew = openApiSpec.paths['/api/crew'].get.responses[200].content['application/json'].schema;
     expect(crew.type).toBe('array');