[v0.20.x-branch] Backport #11106: invoices: migrate legacy AMP invoice HTLCs

[github-actions]

Backport of #11106

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Change Description

Legacy AMP invoices created before reusable AMP invoices store their HTLCs
inline and don't contain AMPState metadata.

The KV-to-SQL migration inserted the generic HTLC rows for these invoices, but
without AMPState it did not create AMP sub-invoices or associate the HTLCs
with them. SQL read-back therefore returned an empty HTLC map, and strict
migration verification aborted startup.

Reconstruct the modern AMP sub-invoice state from each inline HTLC's set ID
before insertion. This preserves the HTLC associations, AMP metadata, custom
records, amounts, and settlement metadata while keeping the compatibility
handling isolated to the migration.

The migration fails closed if the legacy data is structurally inconsistent.
Release notes are included for v0.21.3 and v0.20.4.

Steps to Test

make lint
go vet ./invoices
go test ./invoices -run '^TestMigrateLegacyAMPInvoice -count=20

The regression test migrates a settled legacy AMP invoice with a canceled
competing set, AMP metadata, a large child index, and custom records, then
verifies complete SQL read-back equality.

[github-actions]

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-11106-to-v0.20.x-branch
git worktree add --checkout .worktree/backport-11106-to-v0.20.x-branch backport-11106-to-v0.20.x-branch
cd .worktree/backport-11106-to-v0.20.x-branch
git reset --hard HEAD^
git cherry-pick -x cb9c86195251422dc217c35b915e54a55d8f8580 b7a2f153962c34b3920533da0ae70f05e01d7a4f
git push --force-with-lease

[github-actions]

🟠 PR Severity: HIGH

gh pr view | 3 files | 401 lines changed (117 non-test)

🟠 High (1 file)

• invoices/sql_migration.go - modifies the KV-to-SQL invoice migration logic (reconstructs AMP sub-invoice state), under invoices/*

🟢 Low (2 files)

• docs/release-notes/release-notes-0.20.4.md - release notes only
• invoices/sql_migration_test.go - test-only change

Analysis

This is a backport of #‌11106 fixing a bug in the KV-to-SQL migration for legacy AMP invoices: without reconstructed AMPState, HTLCs weren't associated with AMP sub-invoices, causing empty HTLC maps and failed strict migration verification on startup. The core logic change is confined to invoices/sql_migration.go, which falls under invoices/* (HIGH severity). It does not touch channeldb/migration*, sqldb/*, or wtdb/*, so the always-CRITICAL database-migration rule doesn't apply. Non-test/non-doc changes total only ~117 lines across a single file, well under the thresholds for a severity bump. Given this touches invoice migration correctness (data integrity on startup), a knowledgeable engineer review is warranted.

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_{To override, add a severity-override-{critical,high,medium,low} label.}

[ziggie1984]

LGTM