feat: add SUBLINK_FIXED_SHORT_CODE env var for persistent short links

When SUBLINK_FIXED_SHORT_CODE is set, short links always use that code
instead of generating a random one each time, so subscription URLs
don't change after config updates.

Behaviour:
- env var set → fixed short code (KV key overwritten on each save)
- env var unset → random short code (unchanged behaviour)

Compatible with Cloudflare Workers, Pages, and Vercel deployments.

Author: yingxiaomo

src/app/createApp.jsx

@@ -23,7 +23,10 @@ const DEFAULT_USER_AGENT = 'curl/7.74.0';
 export function createApp(bindings = {}) {
     const runtime = normalizeRuntime(bindings);
     const services = {
-        shortLinks: runtime.kv ? new ShortLinkService(runtime.kv, { shortLinkTtlSeconds: runtime.config.shortLinkTtlSeconds }) : null,
+        shortLinks: runtime.kv ? new ShortLinkService(runtime.kv, {
+            shortLinkTtlSeconds: runtime.config.shortLinkTtlSeconds,
+            fixedShortCode: runtime.config.fixedShortCode
+        }) : null,
         configStorage: runtime.kv ? new ConfigStorageService(runtime.kv, { configTtlSeconds: runtime.config.configTtlSeconds }) : null
     };
 

src/runtime/cloudflare.js

@@ -5,6 +5,8 @@ export function createCloudflareRuntime(env) {
         kv: env?.SUBLINK_KV ? new CloudflareKVAdapter(env.SUBLINK_KV) : null,
         assetFetcher: env?.ASSETS ? (request) => env.ASSETS.fetch(request) : null,
         logger: console,
-        config: {}
+        config: {
+            fixedShortCode: env?.SUBLINK_FIXED_SHORT_CODE || null
+        }
     };
 }

src/runtime/runtimeConfig.js

@@ -13,6 +13,7 @@
  * @typedef {Object} RuntimeConfig
  * @property {number} [configTtlSeconds]
  * @property {number} [shortLinkTtlSeconds]
+ * @property {string} [fixedShortCode]
  */
 
 /**
@@ -40,7 +41,8 @@ export function normalizeRuntime(runtime = {}) {
         logger: runtime.logger ?? console,
         config: {
             configTtlSeconds: runtime.config?.configTtlSeconds ?? DEFAULTS.configTtlSeconds,
-            shortLinkTtlSeconds: runtime.config?.shortLinkTtlSeconds ?? null
+            shortLinkTtlSeconds: runtime.config?.shortLinkTtlSeconds ?? null,
+            fixedShortCode: runtime.config?.fixedShortCode ?? null
         }
     };
 }

src/runtime/vercel.js

@@ -11,7 +11,8 @@ export function createVercelRuntime(env = process.env) {
         logger: console,
         config: {
             configTtlSeconds: undefined,
-            shortLinkTtlSeconds: null
+            shortLinkTtlSeconds: null,
+            fixedShortCode: env?.SUBLINK_FIXED_SHORT_CODE || null
         }
     };
 }

src/services/shortLinkService.js

@@ -16,7 +16,8 @@ export class ShortLinkService {
 
     async createShortLink(queryString, providedCode) {
         const kv = this.ensureKv();
-        const shortCode = providedCode || generateWebPath();
+        // Fixed short code from config takes priority; then API-provided code; then random
+        const shortCode = this.options.fixedShortCode || providedCode || generateWebPath();
         const ttl = this.options.shortLinkTtlSeconds;
         const putOptions = ttl ? { expirationTtl: ttl } : undefined;
         await kv.put(shortCode, queryString, putOptions);

wrangler.toml

@@ -8,5 +8,8 @@ kv_namespaces = [
   { binding = "SUBLINK_KV", id = "8943789ed0af49659f887fc740d11240" }
 ]
 
+[vars]
+SUBLINK_FIXED_SHORT_CODE = ""
+
 [assets]
 directory = "./public"