feat: bitcoin pruned mode with guarded seed restores

[erubboli]

What

Bitcoin node storage mode becomes first-class instead of an install-time footnote:

• Sidecar /status now reports pruned, pruneHeight and txindex from getblockchaininfo (shapes unit-tested).
• Restore guard (create-time): a full-history seed restore (sync horizon 0) on a pruned node is refused with a plain-English 409 instead of silently missing coins. Generated wallets (horizon=now) are unaffected.
• Restore guard (sync-time backstop): if bitcoind was unreachable during the restore attempt and comes back pruned, the sync task refuses a horizon-0 scan — no wrong balances, loud log line.
• link-seed fail-fast: the API checks pruned state before unlocking private keys, so password + 2FA aren't burned on a restore that cannot work.
• UI: "pruned node" chip in the Bitcoin page status strip with a tooltip explaining the limitation; setup/import error paths surface the refusal copy verbatim.
• Docs: README storage-mode matrix (full ~700 GB vs pruned ~15–25 GB + restore limitation; the outdated "pruning incompatible with wallet sync" warning is gone), env.dev.example documents BITCOIN_TXINDEX=0 + BITCOIN_PRUNE=550 for light dev setups.

./init.sh already asks for the mode at install time (pruned by default); this PR makes the runtime stack honest about it.

Gates

• cargo check + sidecar tests (9) green
• tsc --noEmit clean, 74 files / 879 tests green, astro build OK
• Diff reviewed in a recursive findings loop to zero findings; fix at each cycle