#120 [High] Require mandatory code-owner review on security-sensitive paths via branch protection

[james2177]

Description: Building on the CODEOWNERS file added in issue #101, this issue enables and documents GitHub's "require review from Code Owners" branch protection setting specifically scoped to the highest-risk paths — src/soroban/ (on-chain signing/submission), src/common/stellar-signature.ts (signature verification), prisma/schema.prisma (data model), and src/soroban/signer.service.ts (key handling) — so PRs touching these can't merge without a qualified reviewer's sign-off, independent of whatever general review the PR already received.

Problem Statement & Context: This backlog already documents a long history of exactly the class of bug that slips through general review in these paths — signature-verification gaps (issues #82, #83, #96, #97), key-material handling (issue #91), and spoofable trust boundaries (issue #20). A general CODEOWNERS mapping (issue #101) routes review requests, but without branch-protection enforcement it's advisory only; this issue makes review on the highest-risk subset mandatory, not optional.

Scope & Acceptance Criteria:

• Enable "Require review from Code Owners" in the repository's branch protection rules for main, scoped via the CODEOWNERS entries for src/soroban/, src/common/stellar-signature.ts, and prisma/schema.prisma added/refined for this purpose.
• Document the policy and the rationale (linking the specific prior-issue history above) in SECURITY.md (issue Add unit tests for LoggingInterceptor #103).
• Out of scope: building any new tooling — this issue is a CODEOWNERS refinement plus a repo-admin branch-protection configuration change, documented for auditability.

Implementation Guidelines:

1. Key files: CODEOWNERS (from issue Add unit tests for StatsService.getProtocolStats() #101), SECURITY.md (from issue Add unit tests for LoggingInterceptor #103), src/soroban/, src/common/stellar-signature.ts, prisma/schema.prisma.
2. Verify the general CODEOWNERS mapping from issue Add unit tests for StatsService.getProtocolStats() #101 doesn't already imply broader ownership that conflicts with a tighter security-specific rule — reconcile any overlap explicitly rather than leaving two contradictory ownership claims.
3. Confirm the designated code owners for these paths are realistic (actual active maintainers, not a bottleneck of one person) to avoid the mandatory-review requirement stalling contributor PRs indefinitely.
4. Testing/validation: N/A (GitHub configuration, not application code), but the PR implementing this must include a screenshot or exported branch-protection-rule JSON as evidence the setting is live, plus a test PR touching src/soroban/ demonstrating the review requirement actually blocks merge without code-owner approval.

Definition of Done:

• Branch protection configured and scoped correctly, documented in SECURITY.md.
• Acceptance criteria met, including live-verification evidence.
• PR passes CI, reviewed and approved.

Resources: CODEOWNERS, SECURITY.md, src/soroban/, src/common/stellar-signature.ts, prisma/schema.prisma

Complexity: High (200 points)