Predictive Subscription Lifecycle & Automated Renewal Engine

[SatyamPandey-07]

Objective: Refactor the current basic subscription list into a dynamic state-machine engine that manages trials, grace periods, and predicts cash-flow impact.

L3 Complexity: Requires a major rewrite of the subscription service, a new background worker for state transitions, and complex predictive math.

Files involved (8):
models/Subscription.js
(Add state machine & tracking)
services/subscriptionService.js
(Complete refactor)
jobs/renewalWorker.js (New: Handle trials/expiries)
middleware/subscriptionValidator.js
(New: Lifecycle validation)
utils/predictiveMath.js (New: Forecast cash flow impact)
routes/subscriptions.js
(Update for lifecycle actions)
tests/subscription.test.js (Full lifecycle test suite)
SUBSCRIPTION_ENGINE_DOCUMENTATION.md (Technical spec)

[github-actions]

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Meanwhile make sure you've visited the README.md, CONTRIBUTING.md, and CODE_OF_CONDUCT.md before creating a PR for this. Also, please do NOT create a PR until this issue has been assigned to you. 😊