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>

.env.example

@@ -19,3 +19,9 @@ SMTP_FROM=noreply@mindsift.local
 SMTP_TLS=true
 # Only for internal mail servers with self-signed certificates
 SMTP_SKIP_VERIFY=false
+
+# Optional legal notices: external link and/or Markdown (inline or as file path)
+LEGAL_IMPRINT_URL=
+LEGAL_PRIVACY_URL=
+LEGAL_IMPRINT_CONTENT_FILE=
+LEGAL_PRIVACY_CONTENT_FILE=