[v0.21.x-branch] Backport #10982: paymentsdb: normalize orphaned blinded total

[github-actions]

Backport of #10982

---

Change Description

Fix a KV-to-SQL payment migration failure for historical routes containing a
non-zero blinded total_amt_msat without encrypted recipient data.

SendToRouteV2 historically copied the blinded total amount independently of
the encrypted data. LND did not classify such a hop as blinded at runtime, but
the migration treated the total amount as proof of a blinded hop and attempted
to insert a row with a NULL encrypted_data value. This violated the existing
SQL NOT NULL constraint and prevented LND from starting.

This PR:

• requires encrypted recipient data when SendToRouteV2 supplies either a
  blinding point or blinded total amount;
• omits the orphaned total when migrating the historically accepted total-only
  representation;
• rejects malformed blinding-point-only migration records with payment,
  attempt and hop context;
• applies the same normalization during KV/SQL migration validation;
• emits a PYDB warning identifying the affected payment, attempt, hop and
  discarded total amount;
• keeps the strict SQL schema unchanged.

Steps to Test

make unit-debug log="stdlog trace" \
  pkg=lnrpc/routerrpc \
  case=TestUnmarshallHopBlindedFieldsRequireEncryptedData \
  timeout=10s nocache=1

go test -v -tags="dev test_db_sqlite stdlog trace" \
  ./payments/db/migration1 \
  -run '^(TestMigrateOrphanedBlindedTotalAmount|TestMigrateBlindingPointWithoutEncryptedData) \
  -count=1 -timeout=10s

make unit pkg=lnrpc/routerrpc timeout=5m nocache=1

go test -tags=test_db_sqlite ./payments/db/migration1 \
  -count=1 -timeout=5m

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative error paths are included.
• The bug fix contains a regression test triggering the historical case.

Code Style and Documentation

• The change follows the documentation, commenting and 80-column rules.
• Commits follow the ideal Git commit structure.
• New logging uses the PYDB subsystem at warning level.
• A change description is included in the v0.21.2 release notes.

[ziggie1984]

LGTM (no merge conflicts)