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

flake.nix

@@ -3,13 +3,12 @@
 
 	inputs = {
 		nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-		nix-bitcoin.url = "github:fort-nix/nix-bitcoin/release";
 		nixvim.url = "github:nix-community/nixvim";
 		btc-clients.url = "github:emmanuelrosa/btc-clients-nix";
 		nixpkgs-stable.url = "github:nixos/nixpkgs/nixos-26.05";
 	};
 
-	outputs = { self, nixpkgs, nix-bitcoin, nixvim, btc-clients, nixpkgs-stable, ... }:
+	outputs = { self, nixpkgs, nixvim, btc-clients, nixpkgs-stable, ... }:
 
 	let
 		overlay-stable = final: prev: {
@@ -18,11 +17,12 @@
 				config.allowUnfree = true;
 			};
 		};
+		overlay-sovran = import ./pkgs/sovran-overlay.nix;
 	in
 	{
 		nixosConfigurations.nixos = nixpkgs.lib.nixosSystem {
 			modules = [
-				{ nixpkgs.hostPlatform = "x86_64-linux"; }
+				{ nixpkgs.hostPlatform = "x86_64-linux"; nixpkgs.overlays = [ overlay-stable overlay-sovran ]; }
 				self.nixosModules.Sovran_SystemsOS
 				./hardware-configuration.nix
 				./role-state.nix
@@ -32,10 +32,9 @@
 
 		nixosConfigurations.sovran_systemsos-iso = nixpkgs.lib.nixosSystem {
 			modules = [
-				{ nixpkgs.hostPlatform = "x86_64-linux"; }
-				({ config, pkgs, ... }: { nixpkgs.overlays = [ overlay-stable ]; })
+				{ nixpkgs.hostPlatform = "x86_64-linux"; nixpkgs.overlays = [ overlay-stable overlay-sovran ]; }
 				./iso/common.nix
-				nix-bitcoin.nixosModules.default
+				./modules/vendor/nix-bitcoin/modules.nix
 				nixvim.nixosModules.nixvim
 			];
 		};
@@ -43,10 +42,10 @@
 		nixosModules.Sovran_SystemsOS = { pkgs, lib, config, ... }: {
 			imports = [
 				({ config, pkgs, ... }: {
-					nixpkgs.overlays = [ overlay-stable ];
+					nixpkgs.overlays = [ overlay-stable overlay-sovran ];
 				})
 				./configuration.nix
-				nix-bitcoin.nixosModules.default
+				./modules/vendor/nix-bitcoin/modules.nix
 				nixvim.nixosModules.nixvim
 			];
 			config = {