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/nix-bitcoin.nix

@@ -0,0 +1,60 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+{
+  options = {
+    nix-bitcoin = {
+      # Kept for compatibility, now simply aliases system pkgs
+      pkgs = mkOption {
+        type = types.attrs;
+        default = pkgs;
+        defaultText = "pkgs";
+        description = "Alias to system pkgs (vendored nix-bitcoin now uses nixpkgs directly).";
+      };
+
+      useVersionLockedPkgs = mkOption {
+        type = types.bool;
+        default = false;
+        description = "Deprecated — vendored modules always use system pkgs.";
+      };
+
+      pkgOverlays = mkOption {
+        internal = true;
+        type = with types; functionTo attrs;
+        default = _: _: {};
+        description = "Deprecated stub.";
+      };
+
+      lib = mkOption {
+        readOnly = true;
+        default = import ./lib.nix lib pkgs config;
+        defaultText = "vendor/nix-bitcoin/lib.nix";
+      };
+
+      torClientAddressWithPort = mkOption {
+        readOnly = true;
+        default = with config.services.tor.client.socksListenAddress;
+          "${addr}:${toString port}";
+        defaultText = "(See source)";
+      };
+
+      torify = mkOption {
+        readOnly = true;
+        default = pkgs.writers.writeBashBin "torify" ''
+          ${pkgs.tor}/bin/torify \
+            --address ${config.services.tor.client.socksListenAddress.addr} \
+            "$@"
+        '';
+        defaultText = "(See source)";
+      };
+
+      runAsUserCmd = mkOption {
+        readOnly = true;
+        default = if config.security.doas.enable
+                  then "doas -u"
+                  else "sudo -u";
+        defaultText = "(See source)";
+      };
+    };
+  };
+}