feat(backend): edit/repost as new ciphertext version

[Olorunfemi20]

Summary

Implements message editing as an append-only operation. An edit never mutates the original row's ciphertext in place; instead it creates a brand-new message carrying fresh ciphertext and per-device envelopes, linked back to the message it replaces through a new nullable self-referential editsMessageId FK on messages. The gateway broadcasts message_edited { originalMessageId, newMessageId } so clients can resolve a thread to its newest version and render an "edited" marker.

closes #190

Changes

• Schema (db/schema.ts): add editsMessageId uuid to messages, a self-referential FK to messages.id with ON DELETE SET NULL (deleting an original does not cascade away its edits). Added the editsMessage / edits self-relation (shared relationName) so Drizzle can resolve the self-join. The inferred Message type now carries the field automatically.
• Migration 0009_message_edits.sql (+ journal entry): ADD COLUMN edits_message_id and the FK constraint. Standard additive DDL that applies cleanly on top of the existing messages table.
• Socket gateway (socket/messaging.ts): new edit_message handler that
  • requires originalMessageId and the client-generated new messageId,
  • rejects empty content,
  • loads the original and restricts editing to the original sender (senderId === caller),
  • links the new message to the root original (original.editsMessageId ?? original.id) so a chain of edits always collapses to one logical message,
  • is idempotent: a retried edit with an already-seen messageId re-acks without inserting again,
  • persists the new message + fresh message_envelopes, then broadcasts new_message (so recipient devices receive the new ciphertext to decrypt) and message_edited.

The handler mirrors the existing send_message flow (idempotency check, envelope fan-out, message_ack, cache invalidation) to stay consistent with the established patterns.

Acceptance criteria

• Edits create a linked new message + fresh envelopes: a new messages row is inserted with editsMessageId set and new message_envelopes rows are written; new_message is broadcast.
• Clients resolve to the newest version per original: edits link back to the root original id and message_edited { originalMessageId, newMessageId } is broadcast for clients to supersede the previous version.
• Edit authorship restricted to the original sender: non-senders receive an error and no write occurs.

Testing

• src/__tests__/messageEdit.test.ts covers: missing ids, empty content, non-sender rejection, linked-new-message creation with new_message + message_edited broadcast, edit-of-an-edit collapsing to the root, and idempotent replay.
• Full backend suite passes (135 tests), tsc --noEmit clean, ESLint clean (no new warnings), Prettier clean.

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