merge: group_key static article grouping + email-gated report PDF

Feature 1 — group_key for static article admin grouping:
- Migration 0020: group_key TEXT column + index on articles table
- _sync_static_articles(): auto-upserts data/content/articles/*.md on
  every /admin/articles load, reads cornerstone → group_key
- _get_article_list_grouped(): COALESCE(group_key, url_path) as group_id,
  so EN/DE static cornerstones pair into one row (pSEO unchanged)

Feature 2 — Email-gated State of Padel report PDF:
- data/content/articles/state-of-padel-q1-2026-{en,de}.md → reports/
- New reports/ blueprint: GET/POST /<lang>/reports/<slug> (email gate),
  GET /<lang>/reports/<slug>/download (PDF serve)
- Premium PDF: full-bleed navy cover, Padelnomics wordmark watermark at
  3.5% opacity (position:fixed, every page), gold/teal accents, Georgia
  headings, WeasyPrint CSS3 (no JS)
- make report-pdf target to build PDFs
- i18n EN + DE (26 keys each, native German via linguistic-mediation)
- /reports added to RESERVED_PREFIXES, data/content/reports/_build/ gitignored

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

Makefile

@@ -2,7 +2,7 @@ TAILWIND_VERSION := v4.1.18
 TAILWIND         := ./bin/tailwindcss
 SOPS_DOTENV      := sops --input-type dotenv --output-type dotenv
 
-.PHONY: help dev init-landing-seeds css-build css-watch \
+.PHONY: help dev init-landing-seeds css-build css-watch report-pdf \
         secrets-decrypt-dev secrets-decrypt-prod \
         secrets-edit-dev secrets-edit-prod \
         secrets-encrypt-dev secrets-encrypt-prod \
@@ -14,6 +14,7 @@ help:
 	@echo "  init-landing-seeds     Create seed landing files for SQLMesh (run once after clone)"
 	@echo "  css-build              Build + minify Tailwind CSS"
 	@echo "  css-watch              Watch + rebuild Tailwind CSS"
+	@echo "  report-pdf             Build market intelligence report PDFs (WeasyPrint)"
 	@echo "  secrets-decrypt-dev    Decrypt .env.dev.sops → .env"
 	@echo "  secrets-decrypt-prod   Decrypt .env.prod.sops → .env"
 	@echo "  secrets-edit-dev       Edit .env.dev.sops in \$$EDITOR"
@@ -47,6 +48,9 @@ css-build: bin/tailwindcss
 css-watch: bin/tailwindcss
 	$(TAILWIND) -i web/src/padelnomics/static/css/input.css -o web/src/padelnomics/static/css/output.css --watch
 
+report-pdf:
+	uv run python web/scripts/build_report_pdf.py
+
 # ── Secrets (SOPS + age) ─────────────────────────────────────────────────────
 # Requires: sops (https://github.com/getsops/sops) + age (https://github.com/FiloSottile/age)
 # Keys config: .sops.yaml