Files Created
1. server/types/berlin-group.ts
 - TypeScript interfaces: BerlinGroupConfig, BerlinGroupHeaders,
 BerlinGroupSessionData
2. server/services/BerlinGroupSignatureService.ts
 - New @service() class that:
 - Loads private key and certificate PEM files from
 VITE_BG_PRIVATE_KEY_PATH and VITE_BG_CERTIFICATE_PATH at
 construction
 - isEnabled() checks if certificates are loaded
 - static isBerlinGroupPath(path) detects "/berlin-group/" in
 request paths
 - generateHeaders(method, body, consentId?) generates required PSD2
 headers:
 Date
 X-Request-ID
 Digest (SHA-256)
 Signature (RSA-SHA256)
 TPP-Signature-Certificate
 PSU headers
 Redirect URIs (POST only)
 Optional Consent-ID
Files Modified
3. server/services/OBPClientService.ts
 - get(), create(), update(), discard() now check
 isBerlinGroupPath() && isEnabled()
 - Routes Berlin Group requests to new private method
 requestWithBerlinGroupHeaders()
 - Merges signature headers with op