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

pkgs/nbxplorer.nix

@@ -0,0 +1,18 @@
+# Fallback nbxplorer package if not in nixpkgs
+# This is rarely needed — nixpkgs-unstable usually has it
+{ lib, buildDotnetModule, fetchFromGitHub, dotnetCorePackages }:
+buildDotnetModule rec {
+  pname = "nbxplorer";
+  version = "2.5.22";
+  src = fetchFromGitHub {
+    owner = "dgarage";
+    repo = "NBXplorer";
+    rev = "v${version}";
+    sha256 = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
+  };
+  projectFile = "NBXplorer/NBXplorer.csproj";
+  nugetDeps = ./nbxplorer-deps.nix; # not needed if using nixpkgs version
+  dotnet-sdk = dotnetCorePackages.sdk_8_0;
+  dotnet-runtime = dotnetCorePackages.aspnetcore_8_0;
+  meta = with lib; { description = "NBXplorer fallback"; license = licenses.mit; };
+}