Remove checker service (#9)

## Summary

- Remove the checker NixOS module, including its systemd service, system user/group, SOPS secret declaration, and nginx virtual host.
- Remove the checker flake input and its now-unreachable lock-file dependency graph.

## Validation

- `tobserver-agent check` — passed all formatting, whitespace, no-build flake, strict NixOS assertion/systemd unit, secret-policy, and Gitleaks checks.
- Focused Nix evaluation confirmed the checker service, user, nginx virtual host, and SOPS secret declaration are absent; `flake.lock` no longer contains the checker input.

## Risks and limitations

- Deployment will stop exposing the checker service and nginx virtual host, but existing runtime state, a mutable system user, DNS records, ACME data, and the encrypted checker secret may remain until a human removes them if desired.

## Manual follow-up

- Review and merge this pull request in Codeberg.
- Deploy manually on Tobserver after merge.
- After deployment, verify the checker unit and virtual host are absent; optionally clean up checker DNS, runtime state, user, ACME data, and the encrypted secret through the appropriate human-managed process.

Reviewed-on: https://codeberg.org/oibot/Tobserver/pulls/9

flake.nix

@@ -3,7 +3,6 @@
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
     sops-nix.url = "github:Mic92/sops-nix";
     sops-nix.inputs.nixpkgs.follows = "nixpkgs";
-    checker.url = "git+https://codeberg.org/oibot/checker.git";
     hermes-agent.url = "github:NousResearch/hermes-agent";
     bitcoin-pulse = {
       url = "git+https://codeberg.org/oibot/BitcoinPulseAPI.git";
@@ -11,7 +10,7 @@
     };
   };
 
-  outputs = { self, nixpkgs, sops-nix, checker, hermes-agent, bitcoin-pulse, ... }:
+  outputs = { self, nixpkgs, sops-nix, hermes-agent, bitcoin-pulse, ... }:
     let
       system = "x86_64-linux";
     in
@@ -20,7 +19,7 @@
         inherit system;
 
         specialArgs = {
-          inherit bitcoin-pulse checker;
+          inherit bitcoin-pulse;
         };
 
         modules = [
@@ -33,7 +32,6 @@
           ./modules/mempool.nix
           ./modules/nextcloud.nix
           ./modules/totap.nix
-          ./modules/checker.nix
           hermes-agent.nixosModules.default
           ./modules/hermes-agent.nix
           ./modules/lnd.nix