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/alembic/versions/2026_04_25_1000-e3f4a5b6c7d8_mandate_origin_id.py

@@ -0,0 +1,27 @@
+"""Mandate origin: replace free-text with FK to duniter_identities.
+
+Revision ID: e3f4a5b6c7d8
+Revises: d91a3c7f8b02
+Create Date: 2026-04-25 10:00:00.000000
+"""
+
+from __future__ import annotations
+
+from alembic import op
+
+revision = "e3f4a5b6c7d8"
+down_revision = "d91a3c7f8b02"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute("ALTER TABLE mandates DROP COLUMN IF EXISTS origin")
+    op.execute("ALTER TABLE mandates ADD COLUMN IF NOT EXISTS origin_id UUID REFERENCES duniter_identities(id)")
+    op.execute("CREATE INDEX IF NOT EXISTS ix_mandates_origin_id ON mandates (origin_id)")
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_mandates_origin_id")
+    op.execute("ALTER TABLE mandates DROP COLUMN IF EXISTS origin_id")
+    op.execute("ALTER TABLE mandates ADD COLUMN IF NOT EXISTS origin TEXT")