Make migrations optional in deployment configuration

[Copilot]

Deployment fails when Application/Migration directory doesn't exist, causing Nix evaluation error followed by Unit migrate.service not found.

Changes

• NixOS options (NixSupport/nixosModules/options.nix): Made migrations nullable (types.nullOr types.path) with default null

• Migrate service (NixSupport/nixosModules/services/migrate.nix): Conditionally define service only when cfg.migrations != null using lib.mkIf

• Deploy script (flake-module.nix): Check service exists before attempting to start it via systemctl list-unit-files

• Documentation (Guide/deployment.markdown): Annotate example configs to indicate migrations line is optional

Usage

Users can now omit the migrations configuration entirely:

services.ihp = {
    domain = "example.com";
    # migrations = ./Application/Migration;  # Optional - omit if not using migrations
    schema = ./Application/Schema.sql;
    fixtures = ./Application/Fixtures.sql;
    sessionSecret = "xxx";
};

Or explicitly set to null:

services.ihp = {
    domain = "example.com";
    migrations = null;  # Explicitly disable migrations
    schema = ./Application/Schema.sql;
    # ...
};

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow deploy without any migration</issue_title>
<issue_description>

As a workaround I've added a dummy empty SQL migrate file

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Allow deploy without any migration #1911

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