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.ts

@@ -454,6 +454,75 @@ export const openApiSpec = {
         },
       },
     },
+    '/api/missions': {
+      get: {
+        summary: 'Mission board',
+        description:
+          "The save's mission contracts — generated deterministically from the seed, always topped up to at least three AVAILABLE. One mission is ACTIVE at a time; completion is checked after jumps and scans (DELIVER on arrival at its target system, EXPLORE after N scans, FIND when a haul contains its kind) and grants the reward into the cargo hold. Auto-initializes a new game when no save exists yet.",
+        operationId: 'getMissions',
+        responses: {
+          200: {
+            description: 'The mission board',
+            content: {
+              'application/json': {
+                schema: { type: 'array', items: { $ref: '#/components/schemas/Mission' } },
+              },
+            },
+          },
+        },
+      },
+    },
+    '/api/missions/{id}/accept': {
+      post: {
+        summary: 'Accept a mission',
+        description:
+          "Makes an AVAILABLE contract the single ACTIVE one (409 while another is active, or when it isn't AVAILABLE). Costs a turn and writes a mission event. Returns the fresh board.",
+        operationId: 'acceptMission',
+        parameters: [
+          {
+            name: 'id',
+            in: 'path',
+            required: true,
+            schema: { type: 'integer' },
+            description: 'missions.id of the contract to accept',
+          },
+        ],
+        responses: {
+          200: {
+            description: 'Fresh mission board after the acceptance',
+            content: {
+              'application/json': {
+                schema: { type: 'array', items: { $ref: '#/components/schemas/Mission' } },
+              },
+            },
+          },
+          400: {
+            description: 'Non-integer id in the path',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/Error' },
+              },
+            },
+          },
+          404: {
+            description: 'Unknown mission id',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/Error' },
+              },
+            },
+          },
+          409: {
+            description: "Mission isn't AVAILABLE, or another mission is already ACTIVE (one at a time)",
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/Error' },
+              },
+            },
+          },
+        },
+      },
+    },
   },
   components: {
     schemas: {
@@ -597,6 +666,22 @@ export const openApiSpec = {
         },
         required: ['techId'],
       },
+      Mission: {
+        type: 'object',
+        properties: {
+          id: { type: 'integer' },
+          type: { type: 'string', enum: ['DELIVER', 'EXPLORE', 'FIND'] },
+          description: { type: 'string', description: "Human-readable objective, e.g. 'Jump to Kepler Drift'" },
+          targetSystemId: { type: ['integer', 'null'], nullable: true, description: 'DELIVER only — destination system (NULL otherwise)' },
+          conditionKind: { type: ['string', 'null'], enum: ['resource', 'part', null], description: 'FIND only — item kind to haul in (NULL otherwise)' },
+          progressTarget: { type: 'integer', description: 'EXPLORE: scans required; 1 for the others' },
+          progress: { type: 'integer', description: 'Scans counted since acceptance; 0/1 for DELIVER/FIND' },
+          rewardKind: { type: 'string', enum: ['resource', 'part'] },
+          rewardQty: { type: 'integer', description: 'Reward quantity (1–3), granted into the cargo hold on completion' },
+          state: { type: 'string', enum: ['AVAILABLE', 'ACTIVE', 'DONE'] },
+        },
+        required: ['id', 'type', 'description', 'targetSystemId', 'conditionKind', 'progressTarget', 'progress', 'rewardKind', 'rewardQty', 'state'],
+      },
       UpgradeRequest: {
         type: 'object',
         properties: {