vendor: replace nix-bitcoin flake input with minimal vendored modules (nixpkgs-only)

- Remove inputs.nix-bitcoin (fort-nix/nix-bitcoin/release) from flake.nix
- Vendor only 6 services actually used by Sovran: bitcoind, electrs,
  lnd (+lndconnect), rtl, btcpayserver, mempool + supporting infra:
  secrets, onion-services/addresses, operator, nodeinfo, security,
  versioning
- All packages now from nixpkgs directly (pkgs.*) — no pinned pkgs
- Keep nix-bitcoin.* option namespace for compatibility
- backups.nix removed: Sovran uses rsnapshot to Second_Drive
  (configuration.nix: hourly/daily to BTCEcoandBackup) — duplicity
  remote backup not needed
- netns-isolation.nix replaced with stub (5 lines): original 365-line
  bridge/iptables/ip-netns broke Caddy/AlbyHub/RTL a year ago and
  is incompatible with nwc-wallets (requires enable=false). Stub
  keeps option valid but warns if enabled.
- Add pkgs/sovran-overlay.nix for gaps only: lndinit + netns-exec stub

modules/vendor/nix-bitcoin/versioning.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+with lib;
+let
+  options.nix-bitcoin.configVersion = mkOption {
+    type = with types; nullOr str;
+    default = null;
+    description = "Vendored stub — no version migration needed.";
+  };
+in {
+  inherit options;
+  config = {};
+}