Update all five learn documents to match the post-audit codebase:
- Replace references to the deleted server-side X3DH/Double Ratchet (`backend/app/core/encryption/`) with their client-side TypeScript counterparts (`frontend/src/crypto/{x3dh,double-ratchet}.ts`).
- Drop mentions of the removed `RatchetState` and `SkippedMessageKey` PostgreSQL models, the server-generated key paths, and the deprecated `send_encrypted_message` server-encryption method. Add an explicit "why we deleted server-side crypto" rationale in 02-ARCHITECTURE.
- Update API endpoint listings: drop `/encryption/initialize-keys`, `/encryption/rotate-signed-prekey`, `/encryption/opk-count`; add `/auth/me`, `/auth/logout`. Note that all non-auth routes require the session cookie.
- Reflect the new schema: `IdentityKey`, `SignedPrekey`, and `OneTimePrekey` now hold only public material; `User` carries a 64-byte `webauthn_user_handle`.
- Replace the `/ws?user_id=...` example with cookie-authenticated WebSocket usage.
- Remove all line-number references throughout (per the no-line-numbers feedback) — ~300 stripped via regex pass plus targeted prose rewrites.
- Update WebAuthn copy to reflect `UserVerificationRequirement.REQUIRED`, the per-credential-backup-eligibility flag, and challenge-bytes-keyed authentication storage.
- Refresh the file index and project tree to match the trimmed module layout.
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