PR-i18n-2b: DE translations for 183/183 catalog keys + test fixes

The DE locale catalog landed empty in PR-i18n-2a (mechanical wrap
only). Production at filemorph.io/de/ rendered English everywhere
because gettext falls back to msgid when msgstr is empty.

This PR translates all 183 keys to German with a technical, terse
tone matching the existing DE templates (impressum, original-DE
enterprise) and the CLAUDE.md "Technology First" motto — no
marketing-speak, direct addressing (du-form), German number
formatting (10.000 not 10,000; 0,05 not 0.05), German tech-term
conventions (Konvertieren, Komprimieren, Tarife, Preise,
Datenschutzerklärung, Nutzungsbedingungen).

Brand names stay English (FileMorph, Pro, Business, Enterprise,
Compliance Edition, GitHub, Stripe). Format/extension lists
(HEIC, JPG, MP4 etc.) stay verbatim. Boilerplate that's a brand
feature stays untranslated (GitHub Security Advisories,
Safe-Harbour, AGPLv3 Open Source).

Test fixes — 6 tests asserted English-only strings on the
unprefixed default route, which now renders DE. Updated to hit
/en/<path> explicitly when the assertion checks an English copy
marker:

- test_verify_email_page_renders → /en/verify-email
- test_each_tier_lists_quota_concurrent_and_rate (pricing_html
  fixture) → /en/pricing  (DE uses 10.000/100.000 + "gleichzeitig")
- test_public_legal_page_reachable[/security-...] → /en/security
  (DE renders "Eine Sicherheitslücke melden")
- test_pricing_renders_coming_soon_when_stripe_disabled → /en/pricing
  (DE renders "demnächst verfügbar")

462 passed, 12 skipped, 0 failed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MrChengLen