docs(15-02): complete multi-signer send route plan

- SUMMARY.md: multi-signer token loop + legacy fallback route rewrite
- STATE.md: advance to plan 3, record metrics, add decisions
- ROADMAP.md: update phase 15 progress (2/3 plans complete)
- REQUIREMENTS.md: mark MSIGN-05, MSIGN-06 complete

.planning/STATE.md

@@ -3,14 +3,14 @@ gsd_state_version: 1.0
 milestone: v1.1
 milestone_name: Smart Document Preparation
 status: executing
-stopped_at: Completed 15-01-PLAN.md — signer-download token pair, sendSignerCompletionEmail, public download route
-last_updated: "2026-04-03T21:44:28.033Z"
+stopped_at: Completed 15-02-PLAN.md — Multi-signer send route with token loop and legacy fallback
+last_updated: "2026-04-03T21:47:51.565Z"
 last_activity: 2026-04-03
 progress:
   total_phases: 17
   completed_phases: 15
   total_plans: 52
-  completed_plans: 49
+  completed_plans: 50
   percent: 100
 ---
 
@@ -25,7 +25,7 @@ See: .planning/PROJECT.md (updated 2026-04-03)
 ## Current Position
 
 Phase: 15 (multi-signer-backend) — EXECUTING
-Plan: 2 of 3
+Plan: 3 of 3
 Status: Ready to execute
 Last activity: 2026-04-03
 
@@ -87,6 +87,7 @@ Progress: [█████████████] 100% (13/13 phases complete
 | Phase 13-ai-field-placement-and-pre-fill P03 | 2 | 2 tasks | 5 files |
 | Phase 14-multi-signer-schema P01 | 5 | 2 tasks | 3 files |
 | Phase 15 P01 | 2 | 3 tasks | 3 files |
+| Phase 15-multi-signer-backend P02 | 5 | 1 tasks | 1 files |
 
 ## Accumulated Context
 
@@ -159,6 +160,8 @@ Recent decisions affecting v1.1 work:
 - [Phase 14-multi-signer-schema]: No Partially Signed enum added — partial state computed dynamically by counting usedAt IS NOT NULL tokens in Phase 16
 - [Phase 15]: signer-download token is 72h with no DB record — same pattern as agent-download, longer TTL for signer latency
 - [Phase 15]: signerEmail stored in DB only (not JWT payload) — keeps token minimal, consistent with D-03
+- [Phase 15-multi-signer-backend]: Promise.all fail-fast for multi-signer send: one email failure rolls back entire send, agent retries — consistent with legacy single-signer behavior
+- [Phase 15-multi-signer-backend]: APP_BASE_URL replaces NEXT_PUBLIC_BASE_URL for signing URLs — server-side env var correct for API routes
 
 ### v1.2 Pre-decisions (from research)
 
@@ -182,6 +185,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-04-03T21:44:28.030Z
-Stopped at: Completed 15-01-PLAN.md — signer-download token pair, sendSignerCompletionEmail, public download route
+Last session: 2026-04-03T21:47:51.561Z
+Stopped at: Completed 15-02-PLAN.md — Multi-signer send route with token loop and legacy fallback
 Resume file: None