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/workflow.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-UI_STATUSES = ("new", "rewrite", "publish", "published", "close")
+UI_STATUSES = ("new", "rewrite", "publish", "published", "close", "no_image")
 
 
 def internal_to_ui_status(status: str | None) -> str:
@@ -11,7 +11,7 @@ def internal_to_ui_status(status: str | None) -> str:
         return "close"
     if value == "review":
         return "rewrite"
-    if value in {"new", "rewrite", "published"}:
+    if value in {"new", "rewrite", "published", "no_image"}:
         return value
     return value or "new"
 
@@ -22,7 +22,7 @@ def ui_to_internal_status(status: str | None) -> str:
         return "approved"
     if value == "close":
         return "error"
-    if value in {"new", "rewrite", "published"}:
+    if value in {"new", "rewrite", "published", "no_image"}:
         return value
     if value in {"approved", "error", "review"}:
         return value
@@ -35,4 +35,5 @@ ALLOWED_UI_TRANSITIONS: dict[str, set[str]] = {
     "publish": {"published", "close"},
     "published": {"rewrite", "close"},
     "close": {"rewrite"},
+    "no_image": {"rewrite", "close"},
 }