Add spdx_identifier or custom_license constraint to software_license_policies

What does this MR do and why?

This MR adds a database constraint to match the model validation. This constraint could not be added before because the software_license_policies in the sec database was not truncated yet as discussed in this thread.

Migration up

main: == 20250819181649 AddSoftwareLicenseOrCustomLicenseConstraintToSoftwareLicensePolicies: migrating 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("ALTER TABLE software_license_policies\nADD CONSTRAINT check_6cb3facbb3\nCHECK ( num_nonnulls(custom_software_license_id, software_license_spdx_identifier) = 1 )\nNOT VALID;\n")
main:    -> 0.0037s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0028s
main: -- execute("ALTER TABLE software_license_policies VALIDATE CONSTRAINT check_6cb3facbb3;")
main:    -> 0.0014s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20250819181649 AddSoftwareLicenseOrCustomLicenseConstraintToSoftwareLicensePolicies: migrated (0.0694s) 

Migration down

main: == [advisory_lock_connection] object_id: 154160, pg_backend_pid: 18911
main: == 20250819181649 AddSoftwareLicenseOrCustomLicenseConstraintToSoftwareLicensePolicies: reverting 
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- execute("            ALTER TABLE software_license_policies\n            DROP CONSTRAINT IF EXISTS check_6cb3facbb3\n")
main:    -> 0.0016s
main: == 20250819181649 AddSoftwareLicenseOrCustomLicenseConstraintToSoftwareLicensePolicies: reverted (0.0244s) 

References

Related to #550228 (closed)

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