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/models.py

@@ -43,13 +43,14 @@ class RemissionPractice(Base):
     """
     __tablename__ = "remission_practice"
     __table_args__ = (
-        UniqueConstraint("application_id", name="uq_remission_practice_application"),
+        UniqueConstraint("application_id", "sequence_number",
+                         name="uq_remission_practice_app_seq"),
         {"schema": "gepafin_rendic"},
     )
 
     id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
     call_id = Column(Integer, nullable=False)
-    application_id = Column(Integer, nullable=False, unique=True)
+    application_id = Column(Integer, nullable=False)  # unique (application_id, sequence_number)
     company_id = Column(Integer, nullable=False)
     user_id = Column(Integer, nullable=False)  # beneficiario che compila
 
@@ -61,6 +62,11 @@ class RemissionPractice(Base):
     amount_erogato = Column(Numeric(14, 2), nullable=False)  # copiato da application.amount_accepted
     notes_beneficiario = Column(Text, nullable=True)
 
+    # Multi-tranche v2 (2026-04-18)
+    sequence_number = Column(Integer, nullable=False, default=1)
+    period_label = Column(String(100), nullable=True)  # libero, es "I trimestre 2021"
+    suggested_instructor_id = Column(Integer, nullable=True)  # letto da BE assigned_applications
+
     # colonne istruttoria
     assigned_instructor_id = Column(Integer, nullable=True)
     reviewed_at = Column(DateTime(timezone=True), nullable=True)
@@ -80,6 +86,7 @@ class RemissionPractice(Base):
     ula_employees = relationship("RemissionUlaEmployee", back_populates="practice", cascade="all, delete-orphan")
     documents = relationship("RemissionDocument", back_populates="practice", cascade="all, delete-orphan")
     amendment_requests = relationship("RemissionAmendmentRequest", back_populates="practice", cascade="all, delete-orphan")
+    custom_checks = relationship("RemissionCustomCheckValue", back_populates="practice", cascade="all, delete-orphan")
 
 
 class RemissionInvoice(Base):
@@ -227,3 +234,47 @@ class RemissionAmendmentRequest(Base):
     updated_at = Column(DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now())
 
     practice = relationship("RemissionPractice", back_populates="amendment_requests")
+
+
+class RemissionCustomCheckValue(Base):
+    """Valore di un controllo custom configurato dallo schema del bando.
+    Schema custom_checks[] nel template definisce code/label/description/requires_document/required.
+    Qui salviamo dichiarazione beneficiario + eventuale documento + verifica istruttore.
+    """
+    __tablename__ = "remission_custom_check_value"
+    __table_args__ = (
+        UniqueConstraint("practice_id", "check_code", name="uq_custom_check_practice_code"),
+        {"schema": "gepafin_rendic"},
+    )
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    practice_id = Column(UUID(as_uuid=True),
+                         ForeignKey("gepafin_rendic.remission_practice.id", ondelete="CASCADE"),
+                         nullable=False)
+    check_code = Column(String(64), nullable=False)  # es "antiriciclaggio", "polizza_fidejussoria"
+
+    # Dichiarazione beneficiario
+    beneficiary_declared = Column(Boolean, nullable=False, default=False)
+    declared_at = Column(DateTime(timezone=True), nullable=True)
+
+    # Documento allegato (se requires_document)
+    storage_path = Column(String(1024), nullable=True)
+    mime = Column(String(128), nullable=True)
+    size_bytes = Column(BigInteger, nullable=True)
+    sha256 = Column(String(64), nullable=True)
+    document_uploaded_at = Column(DateTime(timezone=True), nullable=True)
+    uploaded_by = Column(Integer, nullable=True)
+
+    # Verifica istruttore
+    verification_status = Column(String(20), nullable=False, default="PENDING")
+    # PENDING | VALIDO | NON_VALIDO
+    verification_notes = Column(Text, nullable=True)
+    verified_by = Column(Integer, nullable=True)
+    verified_at = Column(DateTime(timezone=True), nullable=True)
+
+    created_at = Column(DateTime(timezone=True), nullable=False, server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), nullable=False,
+                        server_default=func.now(), onupdate=func.now())
+
+    practice = relationship("RemissionPractice", back_populates="custom_checks")
+