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

prautoblogger.php

@@ -53,6 +53,19 @@ define( 'PRAUTOBLOGGER_DEFAULT_ANALYSIS_MODEL', 'google/gemini-2.5-flash-lite' )
 define( 'PRAUTOBLOGGER_DEFAULT_WRITING_MODEL', 'google/gemini-2.5-flash-lite' );
 define( 'PRAUTOBLOGGER_DEFAULT_EDITOR_MODEL', 'google/gemini-2.5-flash-lite' );
 
+// Image generation defaults (Cloudflare Workers AI, FLUX family).
+// Short alias; the Cloudflare image pricing helper expands it to the full
+// `@cf/black-forest-labs/...` model id when building the Workers AI URL.
+define( 'PRAUTOBLOGGER_DEFAULT_IMAGE_MODEL', 'flux-1-schnell' );
+
+// CEO-approved image style suffix (decision locked 2026-04-15). Users may
+// override via Settings → Images → Style Suffix; this constant is the
+// fallback the setting reads on first install.
+define(
+	'PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX',
+	'Style: a screengrab from a 1995 late-night cable TV infomercial, oversaturated warm colors, VHS tape noise and scan lines, a bright yellow starburst graphic in the top right corner reading NEW, cheap studio lighting, dated 4:3 television framing, deadpan earnest expression, faint lens distortion, tacky low-budget commercial aesthetic.'
+);
+
 /*
 |--------------------------------------------------------------------------
 | Autoloader