fix(#135): revoke entitlements immediately on plan downgrade

[Nagajyothi-tammisetti]

Program: NSoC'26

Description

Fixes the entitlement leak where downgraded users retained higher-tier
feature access until the next billing cycle boundary. The entitlement
engine now recalculates immediately on plan downgrade via an event-driven
approach with zero coupling between layers.

Changes

File	Role
lib/plans.ts	Single source of truth — tier levels and feature lists
lib/entitlements.ts	Pure helpers — hasFeature, isDowngrade, getEntitlementsForPlan
lib/subscriptions.ts	Emits subscription.downgraded whenever newTier < oldTier
lib/entitlementSync.ts	Subscribes to event, recalculates entitlements immediately
__tests__/entitlementDowngrade.test.ts	Asserts revocation < 1s, upgrade/same-plan don't fire event

Checklist

• My changes fix the issue described in plan downgrade entitlement leak (bug) #135
• I have tested my changes locally
• New test added that asserts revocation within 1 second of downgrade

Closes #135

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❌ PR Validation Failed

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Please edit your PR description to include them, then push a new commit or re-save the description. 📝

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⚠️ @Nagajyothi-tammisetti — your PR body doesn't declare a program (GSSoC/NSoC/General). Please update your description to include the program name. 📝

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⚠️ DCO Sign-off Missing

The following commit(s) are missing a Signed-off-by: line:

• f49b4f4 — fix(plan downgrade entitlement leak (bug) #135): revoke entitlements immediately on plan downgrade

How to fix:

# For the last commit:
git commit --amend --signoff
git push --force-with-lease

# For multiple commits (replace N with the count):
git rebase HEAD~1 --signoff
git push --force-with-lease

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❌ PR Validation Failed

The following required elements are missing:

• Closes #<issue-number> reference
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Please edit your PR description to include them, then push a new commit or re-save the description. 📝

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⚠️ @Nagajyothi-tammisetti — your PR body doesn't declare a program (GSSoC/NSoC/General). Please update your description to include the program name. 📝

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❌ Program Mismatch — Your PR declares NSOC26 but the nsoc26 label is not applied.

Please ensure the correct program label is on your PR, or update your PR description to match. A maintainer can also fix this. 🏷️

[github-actions]

❌ Program Mismatch — Your PR declares NSOC26 but the nsoc26 label is not applied.

Please ensure the correct program label is on your PR, or update your PR description to match. A maintainer can also fix this. 🏷️

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ℹ️ No mentor:* label was found on this PR at merge time. The fallback label mentor:Ayushh-Sharmaa has been applied automatically. If this is incorrect, a maintainer can update it. 🏷️