Add icon support to server.json schema (#620)

Implements icon support based on the Model Context Protocol
specification, allowing servers to specify display icons with optional
size, MIME type, and theme variants.

## Changes

- Add `Icon` type to `pkg/model/types.go` with struct tags for automatic
API validation
- Add `icons` field to `ServerJSON` in `pkg/api/v0/types.go`
- Add icon validation in `internal/validators/validators.go`:
  - Only HTTPS URLs allowed (no HTTP or data URIs)
  - Maximum URL length of 255 characters
  - Validates absolute URLs with proper scheme
- Update JSON schema and OpenAPI documentation with Icon definitions
- Add comprehensive test cases for icon validation
- Add example icon to seed.json (Airtable server)
- Change `websiteUrl` validation to HTTPS-only for consistency

## Implementation Details

The implementation uses a two-layer validation architecture:

1. **Huma framework** validates struct tags (`required`, `maxLength`,
`enum`, `pattern`) and returns 422 for violations
2. **Custom validators** handle business logic (HTTPS scheme
requirement, absolute URL requirement) and return 400 for violations

This approach leverages Huma's automatic validation while keeping
business-specific rules in the validator layer.

## Icon Field Specification

- `src` (required): HTTPS URL to icon resource, max 255 characters
- `mimeType` (optional): One of `image/png`, `image/jpeg`, `image/jpg`,
`image/svg+xml`, `image/webp`
- `sizes` (optional): Array of size strings in "WxH" format or "any" for
scalable formats
- `theme` (optional): Either "light" or "dark" to indicate intended
background

Multiple icons can be provided (e.g., for different themes or sizes).

internal/validators/constants.go

@@ -38,3 +38,7 @@ const (
 	SourceGitHub RepositorySource = "github"
 	SourceGitLab RepositorySource = "gitlab"
 )
+
+const (
+	SchemeHTTPS = "https"
+)