feat(prospecting): add content scraping for POC builder (Workstream 3A)

- New scrape_content() method in enrichment_service: extracts meta
  description, H1/H2 headings, paragraphs, images (filtered for size),
  social links, service items, and detected languages using BeautifulSoup
- Scans 6 pages per prospect: /, /about, /a-propos, /services,
  /nos-services, /contact
- Results stored as JSON in prospect.scraped_content_json
- New endpoints: POST /content-scrape/{id} and /content-scrape/batch
- Added to full_enrichment pipeline (Step 5, before security audit)
- CONTENT_SCRAPE job type for scan-jobs tracking
- "Content Scrape" batch button on scan-jobs page
- Add beautifulsoup4 to requirements.txt

Tested on batirenovation-strasbourg.fr: extracted 30 headings,
21 paragraphs, 13 images.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/prospecting/static/admin/js/scan-jobs.js

@@ -53,6 +53,7 @@ function scanJobs() {
             'tech_scan': 'tech-scan',
             'performance_scan': 'performance',
             'contact_scrape': 'contacts',
+            'content_scrape': 'content-scrape',
             'security_audit': 'security-audit',
             'score_compute': 'score-compute',
         },