Add concurrent subscription update conflict resolution with OCC

[Smartdevs17]

Context

When multiple admin users or automated processes operate on the same subscription simultaneously, last-writer-wins causes silent data loss.

Current Limitation/Problem

A support agent updating subscription notes can be overwritten by billing's status change. No conflict detection exists.

Expected Outcome

Optimistic Concurrency Control (OCC) using version numbers: each entity has a version field incremented on write. Updates include WHERE version = N and fail with 409 on conflict.

Acceptance Criteria

• Version field: add integer version column to subscription, invoice, plan entities (default 1)
• Optimistic update: UPDATE ... WHERE id = $1 AND version = $2 returning rowcount (0 = conflict)
• Conflict response: 409 Conflict with current version in response body
• Client retry: fetch latest entity version and retry mutation (up to 3 attempts)
• Conflict notification: show diff UI when automatic retry exhausted after 3 attempts
• Mutation history: log conflicting writes with field-level diff for audit
• Edge case: bulk operations increment version individually per row
• Edge case: admin bypass parameter (?force=true) for emergency overrides (logged)

Technical Scope

• db/migrations/ - add version column to subscription, invoice, plan tables
• backend/shared/occ/ - OptimisticLockService (version check + increment)
• backend/subscription/controller/ - version validation in update endpoints
• backend/billing/controller/ - version validation in billing operations
• mobile/app/services/ - conflict retry logic with exponential backoff

[Tekprecious]

@Tekprecious has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I have experience for this and have reviewed the code; I can submit a PR within 2 days.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Tekprecious to this issue.

[coderolisa]

@coderolisa has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer let me work on this
please assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @coderolisa to this issue.

[abdulqoyumadegoke]

@abdulqoyumadegoke has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this task, kindly assign it to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @abdulqoyumadegoke to this issue.

[Marvy247]

@Marvy247 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer, I've reviewed the issue description, and I'm confident I can implement all the requested details smoothly.​ Please assign this issue to me! I can have a PR ready for review in a few hours. Thank You.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Marvy247 to this issue.

[drips-wave]

Congratulations, @Tekprecious! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Tekprecious: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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