feat(v2): multi-tranche DB schema + gate cumulativo 5 voci Cecilia

A1 migrations.py:
- remission_practice DROP uq_application + ADD sequence_number/period_label/suggested_instructor_id
- UNIQUE composita (application_id, sequence_number)
- partial index idx_remission_practice_unassigned su assigned_instructor_id NULL
- nuova tabella remission_custom_check_value (storage_path/mime/size/sha256 allineata adapter)

A2 models.py + templates.py:
- RemissionPractice: UniqueConstraint composita, campi multi-tranche, relationship custom_checks
- classe RemissionCustomCheckValue
- RESTART_TEMPLATE schema_version=2, max_tranches=2, custom_checks esempio
  (antiriciclaggio required no-doc, polizza_fidejussoria optional con-doc)
- upgrade_schema_to_v2 idempotente per snapshot v1 esistenti

A3 _compute_gate_check(db, practice) CUMULATIVO:
- max_remission_global = min(cap_pct * erogato, cap_abs)
- already_approved = func.sum(approved_remission) su tranche APPROVED precedenti
  dello stesso application_id con sequence_number < corrente
- max_remission_this_tranche = max(0, global - already_approved)
- pre_check_admissible = min(grand_total_declared, this_tranche)  [voce 2 Cecilia]
- remission_due = min(effective_total, this_tranche)
- residuo_da_restituire = erogato - already_approved - remission_due (cumulativo)
- output totals esteso: sequence_number, tranches_count, tranches_max
- signature (db, practice) - aggiornati 6 call site in practices/instructor/verbale

Test su NAPOLI SAS: erogato 17K, cap 8500, tranche 1 approvata 467.14EUR,
tranche 2 vuota -> residuo disponibile 8032.86EUR, residuo_da_restituire 16532.86EUR.

app/routers/verbale.py

@@ -106,7 +106,7 @@ def _is_instructor(user: AuthUser) -> bool:
 def _build_context(db: Session, practice: RemissionPractice, user: AuthUser) -> dict:
     """Prepara tutto il contesto per il template."""
     # Gate check + totali
-    gate_obj = _compute_gate_check(practice); gate = gate_obj.model_dump() if hasattr(gate_obj, "model_dump") else dict(gate_obj)
+    gate_obj = _compute_gate_check(db, practice); gate = gate_obj.model_dump() if hasattr(gate_obj, "model_dump") else dict(gate_obj)
     totals = gate.get("totals") or {}
 
     # Schema sections