feat(images): Cloudflare Workers AI image provider (commit 1a)

First commit of the image + Instagram A/B pipeline workstream. Ships the
Cloudflare Workers AI image provider, its pricing + validator helpers,
four new settings fields in a new "Images" admin section, and unit tests
with mocked HTTP. Nothing calls the provider yet from the article
pipeline — integration lands in commit 1b.

Why: CEO-approved on 2026-04-15 (see
convo/prautoblogger/threads/2026-04-image-pipeline/01-cto-handoff.md).
Style is locked to the 90s infomercial pastiche; default model is FLUX.1
[schnell] at ~$0.0011/MP (~$0.01/article for 8 images across 4 placements
x 2 variants).

Risk flags:
- Direct call to Cloudflare Workers AI bypassing AI Gateway (decision
  D-001, documented in convo/prautoblogger/decisions/). Gateway route
  to Workers AI currently 403s; OpenRouter gateway path unaffected.
- prab_event_log structured writes deferred to commit 1b when the
  table lands (decision D-002). Logger-based logging is in place now.
- No schema changes. No existing code paths touched outside settings.
- Backward compatible: provider is instantiable but not wired to any
  consumer yet.

Test plan:
- CI runs PHPUnit including the new CloudflareImageProviderTest.
- After merge: verify the new Images section appears on the settings
  page, the style-suffix textarea pre-fills with the infomercial prompt,
  and entering the existing CLOUDFLARE_AI_TOKEN + account UUID lets a
  future "Test Connection" button return OK (button ships in a later
  commit; the underlying validate_credentials_detailed() works today).

Files:
- includes/providers/interface-image-provider.php   (adopted from CTO draft)
- includes/providers/class-cloudflare-image-provider.php  (283 lines)
- includes/providers/class-cloudflare-image-pricing.php   (78 lines)
- includes/providers/class-cloudflare-image-validator.php (107 lines)
- includes/admin/class-settings-fields.php (+ Images section, 4 fields)
- prautoblogger.php (+ PRAUTOBLOGGER_DEFAULT_IMAGE_MODEL, _STYLE_SUFFIX)
- tests/unit/Providers/CloudflareImageProviderTest.php
- ARCHITECTURE.md (file tree, integrations table, options table, decision #16)
- CONVENTIONS.md (How To: Add a New Image Provider)
- CHANGELOG.md (Unreleased section)

Agent-Session: cowork-2026-04-15-image-pipeline-commit-1a

CONVENTIONS.md

@@ -122,6 +122,34 @@ interface PRAutoBlogger_Source_Provider_Interface {
 
 ---
 
+## How To: Add a New Image Provider
+
+1. Create `includes/providers/class-{name}-image-provider.php`
+2. Implement `PRAutoBlogger_Image_Provider_Interface` (see `interface-image-provider.php`)
+3. Required methods: `generate_image()`, `estimate_cost()`, `get_provider_name()`, `validate_credentials_detailed()`
+4. Split helpers into companions if you'd exceed the 300-line cap. The Cloudflare provider splits into provider + pricing + validator — mirror that shape.
+5. Add API credentials fields to `admin/class-settings-fields.php` under the `prautoblogger_images` section
+6. Register the provider in `class-prautoblogger.php` loader once the image pipeline lands (commit 1b of the image workstream)
+7. Update `ARCHITECTURE.md` file tree, external API integrations table, and the options table
+
+### Interface contract:
+```php
+interface PRAutoBlogger_Image_Provider_Interface {
+    public function generate_image( string $prompt, int $width, int $height, array $options = [] ): array; // {bytes, mime_type, width, height, model, seed, cost_usd, latency_ms}
+    public function estimate_cost( int $width, int $height, array $options = [] ): float;
+    public function get_provider_name(): string;                                   // e.g., 'cloudflare_workers_ai'
+    public function validate_credentials_detailed(): array;                         // {status, message, debug?}
+}
+```
+
+### Retry + cost rules (non-negotiable)
+- 5xx / 429 / network errors: retry with exponential backoff up to `PRAUTOBLOGGER_MAX_RETRIES`, respect `Retry-After` when present.
+- 4xx other than 429: fail loudly on the first response — never retry a deterministic bad request.
+- Validation check must be non-destructive (no image generation).
+- Every attempt logs via `PRAutoBlogger_Logger` with context `{provider-slug}-image`.
+
+---
+
 ## How To: Add a New Admin Setting
 
 1. Define the option key in the settings array in `admin/class-admin-page.php` → `get_settings_fields()` method