feat: forward writes from passive regions to active #691

[Fading-Dev]

Overview

This PR implements active–passive cross-region write forwarding: passive regions proxy or redirect mutating HTTP requests to the configured active region, with hop-loop protection and a clear failure mode when the active region is unreachable.

Related Issue

Closes #691

Changes

🌍 Region Write Forwarding

• [ADD] internal/middleware/region_router.go

  • Passive-role middleware forwards/redirects non-safe methods to ACTIVE_REGION_URL.
  • Supports proxy and redirect modes via REGION_FORWARD_MODE.
  • Sets X-Region-Hop: 1 and returns 508 on loop detection; 503 when active is unreachable.

• [ADD] internal/middleware/region_router_test.go

  • Coverage for active passthrough, proxy/redirect modes, hop loops, and unreachable active.

• [MODIFY] internal/config/config.go

  • Add REGION_ROLE, ACTIVE_REGION_URL, REGION_FORWARD_MODE, REGION_FORWARD_AUTH_TOKEN.

• [MODIFY] internal/routes/routes.go

  • Wire RegionRouterMiddleware early in the global middleware chain.

• [ADD] docs/CROSS_REGION_WRITE_FORWARDING.md

  • Operator guide for topology roles and forwarding modes.

Verification Results

go test ./internal/middleware/ -run Region -count=1
✅ PASS

Acceptance Criteria	Status
Passive region forwards/redirects writes to active	✅ proxy + redirect modes
Hop loops detected	✅ 508 via X-Region-Hop
Unreachable active fails safely	✅ 503
Active region serves writes locally	✅ Role=active passthrough

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