feat(images): route Cloudflare Workers AI via AI Gateway by default

Direct probe on 2026-04-21 confirmed the Cloudflare AI Gateway
route to Workers AI (`gateway.ai.cloudflare.com/.../workers-ai/@cf/...`)
is no longer 403ing — the upstream regression noted in v0.8.0's
ARCHITECTURE.md #16 and in thread image-pipeline seq 6 has been
resolved. Routing via the gateway gives us the same benefits we
already get for OpenRouter LLM calls: response caching, cost/latency
dashboard, rate limiting.

Changes:

- `Cloudflare_Image_Support::build_endpoint_url()` now prefers the
  gateway URL derived from the existing `prautoblogger_ai_gateway_base_url`
  option. Direct API remains the fallback.
- New private helper `build_gateway_workers_ai_url()` strips the
  OpenRouter path suffix from the configured gateway URL and appends
  `/workers-ai/{model}`. Returns empty string (fallback to direct) when:
  no gateway URL configured, the new toggle is off, or the URL isn't a
  valid `https://gateway.ai.cloudflare.com/...` shape.
- New admin toggle `prautoblogger_cf_image_via_gateway` (default on) in
  the Images section. Operators can disable to force direct API without
  clearing the LLM-path gateway URL.
- ARCHITECTURE.md #16 rewritten to reflect the v0.8.2+ gateway routing.
- CHANGELOG.md v0.8.2 entry under Changed.

Tests (new `CloudflareImageSupportTest`):

- Direct URL when gateway not configured.
- Gateway URL derived correctly when configured + toggle on.
- Toggle off forces direct URL even with gateway configured.
- Malformed gateway URL falls back to direct (defensive).
- `is_nsfw_error()` only matches error code 3030 (pinned separately so
  a tolerance regression fails early).

Cost impact: near-zero. Gateway adds a few ms of latency and enables
response caching — could *reduce* cost on identical prompts within the
configurable cache TTL.

Uninstall: new option covered by existing prautoblogger_% LIKE sweep.

Implements Commit 1b (narrow AI-Gateway-migration scope) per thread
image-pipeline seq 7.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

includes/providers/class-cloudflare-image-support.php

@@ -55,11 +55,26 @@ class PRAutoBlogger_Cloudflare_Image_Support {
 	/**
 	 * Build the Workers AI run URL for a given account + model.
 	 *
+	 * By default routes through the Cloudflare AI Gateway we already use for
+	 * OpenRouter — the gateway route to Workers AI was observed 403ing in
+	 * April 2026 (see ARCHITECTURE.md #16), but manual probe on 2026-04-21
+	 * shows it's now returning 200. Gateway routing gives us response caching,
+	 * cost logging, rate limiting, and provider fallback for free.
+	 *
+	 * Direct-API fallback kicks in automatically when either the
+	 * `prautoblogger_ai_gateway_base_url` option is empty (no gateway
+	 * configured at all) or the new `prautoblogger_cf_image_via_gateway`
+	 * toggle is off (explicit opt-out for if the gateway route regresses).
+	 *
 	 * @param string $account_id Cloudflare account UUID.
-	 * @param string $model      Fully-qualified Workers AI model id.
+	 * @param string $model      Fully-qualified Workers AI model id (e.g. @cf/black-forest-labs/flux-1-schnell).
 	 * @return string            Fully-formed HTTPS URL.
 	 */
 	public function build_endpoint_url( string $account_id, string $model ): string {
+		$gateway_url = $this->build_gateway_workers_ai_url( $account_id, $model );
+		if ( '' !== $gateway_url ) {
+			return $gateway_url;
+		}
 		return sprintf(
 			'https://api.cloudflare.com/client/v4/accounts/%s/ai/run/%s',
 			rawurlencode( $account_id ),
@@ -67,6 +82,43 @@ class PRAutoBlogger_Cloudflare_Image_Support {
 		);
 	}
 
+	/**
+	 * Derive a Workers AI URL rooted at the same AI Gateway we already use
+	 * for OpenRouter. Returns empty string when the gateway isn't configured
+	 * or the admin toggle opts out — the caller falls back to direct API.
+	 *
+	 * The existing `prautoblogger_ai_gateway_base_url` option points at
+	 * `https://gateway.ai.cloudflare.com/v1/{account}/{gateway}/openrouter`
+	 * for the LLM path; we strip the `/openrouter` suffix and append
+	 * `/workers-ai/{model}` for the image path. Defensive: only emits a URL
+	 * that pattern-matches, so a bad option value quietly falls back.
+	 *
+	 * @param string $account_id Cloudflare account UUID (kept for symmetry; not used in URL derivation).
+	 * @param string $model      Fully-qualified Workers AI model id.
+	 * @return string Gateway URL, or empty string if unavailable.
+	 */
+	private function build_gateway_workers_ai_url( string $account_id, string $model ): string {
+		unset( $account_id );
+		if ( '1' !== (string) get_option( 'prautoblogger_cf_image_via_gateway', '1' ) ) {
+			return '';
+		}
+		$gateway_base = rtrim( (string) get_option( 'prautoblogger_ai_gateway_base_url', '' ), '/' );
+		if ( '' === $gateway_base ) {
+			return '';
+		}
+		// Must be an https CF AI Gateway URL.
+		if ( 0 !== stripos( $gateway_base, 'https://gateway.ai.cloudflare.com/' ) ) {
+			return '';
+		}
+		// Strip the trailing OpenRouter path component (any path component, really);
+		// everything before the last '/' is the gateway root shared by all provider routes.
+		$gateway_root = substr( $gateway_base, 0, strrpos( $gateway_base, '/' ) ?: strlen( $gateway_base ) );
+		if ( '' === $gateway_root ) {
+			return '';
+		}
+		return sprintf( '%s/workers-ai/%s', $gateway_root, ltrim( $model, '/' ) );
+	}
+
 	/**
 	 * Emit a WARN line for a retryable failure.
 	 *