chore: remove MVP positioning and align plan defaults (#15)

## Summary
- remove MVP wording from repository docs and guidance
- rename the system plan document and update references to it
- align the default subscription plan code/name with product wording
- document hard subscription expiry with no grace period

## Verification
- docker build -f infra/docker/web.Dockerfile .
- docker build -f infra/docker/migrate.Dockerfile .

Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #15

packages/config/src/index.ts

@@ -153,7 +153,7 @@ function readTelegramMode(env: NodeJS.ProcessEnv, key: string): "polling" {
   const value = readString(env, key);
 
   if (value !== "polling") {
-    throw new Error(`Environment variable ${key} must be "polling" for MVP`);
+    throw new Error(`Environment variable ${key} must be "polling"`);
   }
 
   return value;