docs(15): create phase plan — multi-signer backend (3 plans, 2 waves)

.planning/ROADMAP.md

@@ -333,7 +333,12 @@ Plans:
   3. The POST /api/sign/[token] handler rejects any field submission where the field's signerEmail does not match the token's signerEmail (server-enforced ownership)
   4. When the last signer submits, `completionTriggeredAt` is claimed atomically — only one handler triggers final PDF assembly regardless of concurrent submissions
   5. Agent receives a notification email and all signers receive the final merged PDF link when all signing tokens for the document are claimed
-**Plans**: TBD
+**Plans**: 3 plans
+
+Plans:
+- [ ] 15-01-PLAN.md — Token utility extensions (signerEmail param, signer-download JWT), sendSignerCompletionEmail mailer, public signer download route
+- [ ] 15-02-PLAN.md — Send route rewrite: multi-signer token loop with Promise.all dispatch, legacy single-signer fallback, APP_BASE_URL rename
+- [ ] 15-03-PLAN.md — Sign handler rewrite: GET signer-filtered fields, POST signer-scoped operations + accumulate PDF + atomic completion + notifications
 **UI hint**: no
 
 ### Phase 16: Multi-Signer UI
@@ -345,7 +350,12 @@ Plans:
   2. Each field placed in FieldPlacer has a signer assignment dropdown; selecting a signer changes the field's color to that signer's assigned color
   3. If the agent clicks Send with any client-facing field (signature, initials, date, text) having no signer assigned, the send is blocked and a clear error message identifies the unassigned fields
   4. The dashboard document detail view shows a per-signer completion row for each signer (who has signed, who hasn't, with timestamps)
-**Plans**: TBD
+**Plans**: 3 plans
+
+Plans:
+- [ ] 15-01-PLAN.md — Token utility extensions (signerEmail param, signer-download JWT), sendSignerCompletionEmail mailer, public signer download route
+- [ ] 15-02-PLAN.md — Send route rewrite: multi-signer token loop with Promise.all dispatch, legacy single-signer fallback, APP_BASE_URL rename
+- [ ] 15-03-PLAN.md — Sign handler rewrite: GET signer-filtered fields, POST signer-scoped operations + accumulate PDF + atomic completion + notifications
 **UI hint**: yes
 
 ### Phase 17: Docker Deployment
@@ -358,7 +368,12 @@ Plans:
   3. The `APP_BASE_URL` variable (renamed from `NEXT_PUBLIC_BASE_URL`) is injected at container runtime — signing link URLs in emails contain the correct production domain, not localhost
   4. Uploaded PDF files written inside the container persist after `docker compose down && docker compose up` (named Docker volume mounted at /app/uploads)
   5. The Docker image uses `node:20-slim` (Debian-based) — `@napi-rs/canvas` native binary loads without errors at container startup
-**Plans**: TBD
+**Plans**: 3 plans
+
+Plans:
+- [ ] 15-01-PLAN.md — Token utility extensions (signerEmail param, signer-download JWT), sendSignerCompletionEmail mailer, public signer download route
+- [ ] 15-02-PLAN.md — Send route rewrite: multi-signer token loop with Promise.all dispatch, legacy single-signer fallback, APP_BASE_URL rename
+- [ ] 15-03-PLAN.md — Sign handler rewrite: GET signer-filtered fields, POST signer-scoped operations + accumulate PDF + atomic completion + notifications
 **UI hint**: no
 
 ## Progress
@@ -384,6 +399,6 @@ Phases execute in numeric order: 1 → 2 → 3 → 4 → 5 → 6 → 7 → 8 →
 | 12.1. Per-Field Text Editing and Quick-Fill (INSERTED) | v1.1 | 2/2 | Complete | 2026-03-21 |
 | 13. AI Field Placement and Pre-fill | v1.1 | 3/4 | In Progress | - |
 | 14. Multi-Signer Schema | v1.2 | 1/1 | Complete   | 2026-04-03 |
-| 15. Multi-Signer Backend | v1.2 | 0/TBD | Not started | - |
+| 15. Multi-Signer Backend | v1.2 | 0/3 | Not started | - |
 | 16. Multi-Signer UI | v1.2 | 0/TBD | Not started | - |
 | 17. Docker Deployment | v1.2 | 0/TBD | Not started | - |