Concepts docs: E2EE device identity & key lifecycle (backend)

[codebestia]

Write apps/backend/docs/concepts-e2ee-key-management.md explaining the backend's role in the E2EE key lifecycle: device registration, prekey upload/consumption, key-bundle fetch, revocation, and how the devices/devicePrekeys tables model this.

Acceptance criteria:

• Explains device registration through to revocation as one coherent lifecycle, referencing the actual routes/tables involved by name
• Explains what the server can and cannot see (no private key material ever touches the backend) as an explicit statement
• Cross-references the relevant api-*.md and contracts-*.md docs

[adeboladee]

@adeboladee has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Maintainer, Please I would like to work on this issue

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

[Kingvic300]

@Kingvic300 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this kindly assign it to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kingvic300 to this issue.

[drips-wave]

Congratulations, @Kingvic300! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Kingvic300: 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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