[investify2826-ctrl]

Summary

This PR hardens multi-signature transaction collection and submission across the four requested issues:

• Adds durable MultiSigSignature records with a unique (txId, signerPublicKey) constraint and composite XDR rebuilding so concurrent signer updates cannot erase one another.
• Adds an atomic pending -> submitting compare-and-swap claim so only one concurrent submission can reach Horizon; failed submissions return to pending for retry.
• Adds threshold preflight verification with structured required/current/remaining weight details before Horizon submission.
• Adds configurable multi-sig transaction time bounds, defaulting to seven days, persists the source sequence, detects sequence drift clearly, and supports an optional channel account as the transaction source.
• Adds the Prisma schema and PostgreSQL migration for the new durable concurrency fields.

Validation

• ESLint passed for all modified JavaScript files.
• prisma validate --schema backend/prisma/schema.prisma passed.
• git diff --check passed.
• The existing backend/tests/multiSig.test.js suite was run; it contains legacy hand-rolled Stellar/Prisma mock incompatibilities and unrelated baseline failures under this fork. The repository pre-commit PII scan also reports 14 pre-existing email-like fixtures outside the changed files.

Closes Ethereal-Future#1279
Closes Ethereal-Future#1280
Closes Ethereal-Future#1281
Closes Ethereal-Future#1282