feat(legal): imprint & privacy notices, "Über MindSift"

- LEGAL_IMPRINT_/LEGAL_PRIVACY_ URL and Markdown (inline or *_FILE);
  only http(s) URLs accepted
- /auth/status exposes legal info; public GET /legal/{imprint|privacy}
- Links on login/setup screen and in the settings footer: URL opens a new
  tab, Markdown a reading modal; hidden when nothing is configured
- "Über MindSift" modal (version, license, repository) in settings and help
- Docs and compose/env examples

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

compose.yaml

@@ -27,3 +27,8 @@ services:
       - SMTP_FROM=${SMTP_FROM:-}
       - SMTP_TLS=${SMTP_TLS:-true}
       - SMTP_SKIP_VERIFY=${SMTP_SKIP_VERIFY:-false}
+      # Optional: Impressum / Datenschutz (URL oder Markdown-Datei, z. B. per Volume eingebunden)
+      - LEGAL_IMPRINT_URL=${LEGAL_IMPRINT_URL:-}
+      - LEGAL_PRIVACY_URL=${LEGAL_PRIVACY_URL:-}
+      - LEGAL_IMPRINT_CONTENT_FILE=${LEGAL_IMPRINT_CONTENT_FILE:-}
+      - LEGAL_PRIVACY_CONTENT_FILE=${LEGAL_PRIVACY_CONTENT_FILE:-}