Mandats : origin→FK identité + nomination auto + boutons + tests intégration

- origin TEXT → origin_id UUID FK duniter_identities (migration e3f4a5b6c7d8)
- GET /auth/identities?q= : recherche d'identités par nom/adresse
- MandateCreate.nomination_mode : auto (auto-assign auteur), collective, postpone
- Wizard new.vue : champ origine = picker identité, checkbox "Démarrer maintenant"
- [id].vue : modal "Assigner" = search-picker (résout UUID vs adresse SS58), affiche
  origin_display_name + mandatee_display_name, inputs natifs (<input>/<textarea>)
- Erreurs API visibles dans l'UI (plus de catch silencieux)
- test_mandate_flows.py : 17 tests intégration SQLite réels (origin, nomination,
  assign, lifecycle, revocation, interactions croisées) — 241 tests total OK

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/auth.py

@@ -5,7 +5,8 @@ from __future__ import annotations
 import secrets
 from datetime import datetime, timedelta, timezone
 
-from fastapi import APIRouter, Depends, HTTPException, Response, status
+from fastapi import APIRouter, Depends, HTTPException, Query, Response, status
+from sqlalchemy import or_, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.config import settings
@@ -232,3 +233,24 @@ async def logout(
     for session in sessions:
         await db.delete(session)
     await db.commit()
+
+
+@router.get("/identities", response_model=list[IdentityOut])
+async def search_identities(
+    q: str = Query(..., min_length=1, description="Recherche par adresse ou nom"),
+    limit: int = Query(default=10, ge=1, le=50),
+    db: AsyncSession = Depends(get_db),
+) -> list[IdentityOut]:
+    """Search Duniter identities by address prefix or display_name."""
+    result = await db.execute(
+        select(DuniterIdentity)
+        .where(
+            or_(
+                DuniterIdentity.address.ilike(f"{q}%"),
+                DuniterIdentity.display_name.ilike(f"%{q}%"),
+            )
+        )
+        .order_by(DuniterIdentity.display_name)
+        .limit(limit)
+    )
+    return [IdentityOut.model_validate(i) for i in result.scalars().all()]