sfe: Add rate limit override request portal Web UI and endpoints

[beautifulentropy]

sfe/forms/fields.go:

• Define Web UI field types (input, dropdown, textarea, checkbox) that can be declared and rendered as HTML.

sfe/forms/forms.go:

• Define collections of Web UI fields that can be composed and rendered as HTML.

sfe/overrides.go:

• Define constants and variables used only for fields and values displayed in the Web UI.
• Implement field-level validation for override request form fields.
• Define the forms and fields that make up the override request forms.
• Add HTTP handlers to render forms, validate individual fields on input, and validate complete submissions.

sfe/sfe.go:

• Protect all POST endpoints using https://pkg.go.dev/net/http#CrossOriginProtection (requires go1.25.0 or later).
• Register override form and validation handlers, dependent on a Zendesk client being configured.

sfe/static/overriderequest.js:

• Validate fields as they are filled, block submission until all are valid, send the payload to the server, and redirect on success. All requests use a 10s timeout. Timeouts and general errors are shown in a banner; validation errors are shown beside the field.

Closes #8167

[beautifulentropy]

WebUI.mov