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

docs/plan/system-plan.md

@@ -1,15 +1,15 @@
-# MVP System Plan
+# System Plan
 
 ## Summary
 Build `nproxy`, a B2C web product for image generation through external model APIs. The first model is `nano_banana`. Users register with `email + password`, pay a monthly crypto subscription, receive a monthly request limit, and use a chat-style interface for `text-to-image` and `image-to-image` generation.
 
 The service hides provider-key failures behind a routed key pool. A user request is attempted against one provider key at a time. Retryable failures move execution to the next eligible key. The user sees an error only after all eligible keys have been exhausted or the request fails for a terminal user-caused reason.
 
-## Confirmed MVP decisions
+## Confirmed product decisions
 - One B2C website.
 - One monthly subscription plan.
 - Crypto checkout through a payment processor.
-- Manual renewal in MVP.
+- Manual renewal.
 - Text-to-image and image-to-image.
 - User-facing synchronous experience implemented with polling over background execution.
 - Approximate quota buckets only: `100/80/60/40/20/0`.
@@ -45,7 +45,8 @@ The service hides provider-key failures behind a routed key pool. A user request
 - notifications
 
 ## Billing rules
-- One active plan in MVP.
+- One active plan.
+- Access ends immediately when `currentPeriodEnd` is reached. Warnings may be sent beforehand, but there is no grace period after expiry.
 - Each user has an individual billing cycle based on successful activation timestamp.
 - Limit resets on each successful cycle activation.
 - One successful generation consumes one request.
@@ -60,7 +61,7 @@ Backend tracks exact usage. Normal users see only an approximate bucket:
 - `1-20%` remaining -> `20%`
 - `0%` remaining -> `0%`
 
-## Generation controls in MVP
+## Generation controls
 - mode: `text_to_image` or `image_to_image`
 - resolution preset
 - batch size
@@ -94,8 +95,8 @@ Single VPS with Docker Compose, expected services:
 - `caddy` or `nginx`
 - optional `minio` when object storage is self-hosted
 
-## Future-compatible boundaries
-The codebase should be able to add later:
+## Optional extensions
+These are not current commitments. Consider them only if user demand, operational need, and product scale justify the added complexity:
 - more image providers
 - more billing methods
 - more subscription plans