feat(pipeline): image caption/credit extraction, no-image exclusion, WP attribution

source_extraction.py:
- New _extract_image_metadata(): extracts figcaption text + copyright/credit
  per image URL using 3 strategies (figure+figcaption, data-* attributes,
  adjacent credit spans)
- ExtractedArticle gets new image_metadata field
- extracted_article_to_meta() includes image_metadata in stored JSON

pipeline.py:
- After auto image selection, check if selected_url is set
- Articles without usable image → status "no_image" (excluded with Telegram notice)
- PipelineStats and summary report include no_image counter

db.py:
- Add "no_image" to articles status CHECK constraint
- Migration: recreates articles table with updated constraint on existing DBs

workflow.py / main.py:
- Map no_image as own UI status with rewrite/close transitions

wordpress.py:
- _upload_featured_media() accepts image_caption param, sends to WP media
- _get_image_meta_for_url() / _build_image_caption() helpers
- _build_attribution_block(): separator + attribution paragraph at article end
  (original link, author, Bildnachweis/credit)
- _build_post_content() appends attribution block

telegram_bot.py:
- notify_pipeline_done() shows 🖼️ no-image count

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -123,7 +123,7 @@ class ArticleUpsertRequest(BaseModel):
     publish_last_error: str | None = None
     published_to_wp_at: str | None = None
     word_count: int = 0
-    status: str = Field(default="new", pattern="^(new|rewrite|publish|published|close|review|approved|error)$")
+    status: str = Field(default="new", pattern="^(new|rewrite|publish|published|close|review|approved|error|no_image)$")
     meta_json: str | None = None
 
 
@@ -132,7 +132,7 @@ class IngestionRunRequest(BaseModel):
 
 
 class ArticleTransitionRequest(BaseModel):
-    target_status: str = Field(pattern="^(new|rewrite|publish|published|close|review|approved|error)$")
+    target_status: str = Field(pattern="^(new|rewrite|publish|published|close|review|approved|error|no_image)$")
     note: str | None = None