feat(payments): add payout privacy prototype

[victor-134]

Summary

• add a testnet-only pooled payout option for escrow funding
• default to direct facility payouts in production and local setups
• validate pooled mode with a focused unit test and the existing payments testnet flow

Verification

docforum-core-backend@0.0.0 typecheck
tsc --noEmit

docforum-core-backend@0.0.0 test
vitest run tests/unit --run tests/unit/payment-payout-privacy.test.ts

RUN v4.1.11 /workspaces/docforum-core/backend

Test Files 11 passed (11)
Tests 99 passed (99)
Start at 22:28:43
Duration 4.43s (transform 304ms, setup 0ms, import 834ms, tests 1.87s, environment 1ms)

[drips-wave]

Hey @victor-134! 👋 It looks like this PR isn't linked to any issue.

If this PR is for one of the issues assigned to you as part of a Wave, please link it to ensure your contribution is tracked properly. You can do this by adding a keyword to the PR description (e.g., Closes #123), or by clicking a button below:

Issue	Title
#90	[Phase 5.5] Payment history and facility payout ledger APIs with on-chain proof links	Link to this issue
#91	[Phase 5.5] Nightly testnet payments workflow with automatic Friendbot funding	Link to this issue
#92	[Phase 5.5] ADR + prototype: payout privacy for facilities (ADR 0005 open question)	Link to this issue
#89	[Phase 5.5] Stellar RPC resilience: timeouts, retries, and tracking of in-flight transactions	Link to this issue

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[afeezorobsco-cyber]

@victor-134 Thanks for the PR. It can't be merged yet. To unblock it:

Merge conflicts: this branch conflicts with main. Please rebase onto the latest main (git fetch upstream && git rebase upstream/main), resolve the conflicts (often ROADMAP.md, ARCHITECTURE*.md or prisma/schema.prisma), and force-push.

Push to this same branch and CI will re-run. Once it's green and conflict-free, it'll be reviewed for merge.