wallet: run the node on a SQLite backend (opt-in)

Adds -walletbackend=sqlite: an opt-in flag that runs the node on
<datadir>/wallet.sqlite instead of wallet.dat. It requires an export to
exist first (no silent migration) and prints a step-by-step guide when
one does not; wallet.dat is never opened or touched, so a plain restart
goes back to Berkeley DB.

- LoadWalletFromSQLiteRuntime opens the export read-write, keeps the
  handle open as the active backend, loads records into memory through
  the same visitor/reconcile/finish path as the read-only staging load,
  and never fabricates a default key.
- Shutdown checkpoints and closes the SQLite backend; a kill before that
  loses nothing because WAL + synchronous=FULL already made every
  committed write durable.
- Self-test proves runtime writes persist across a close and reopen.

BDB stays the default: nothing activates the SQLite runtime unless the
flag is given. One-shot audit/recovery commands still use Berkeley DB.

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