feat(remote): add server-CDM mode, manifest transfer, and region-aware proxy

Server-side:
- Add server_cdm mode: server handles full CDM licensing using its own devices, returns KID:KEY pairs instead of raw license bytes
- Support batch license resolution for multiple tracks in one request
- Extract DRM from manifest ContentProtection when track.drm is empty
- Serialize DASH/ISM manifest XML as base64 in /tracks response
- Include session cookies/headers and server_cdm_type in /tracks response
- Detect server CDM type from actual track DRM + configured devices
- Check server region against client_region to skip unnecessary proxy
- Support decrypt_labs and custom_api remote CDMs for both WV and PR

Client-side:
- Re-parse DASH/ISM manifests locally from base64 to populate track.data
- Match remote tracks to re-parsed tracks by ID with attribute fallback
- Copy DRM objects from re-parsed manifests to remote tracks
- Pre-fetch keys via resolve_server_keys() before downloads start
- Fallback per-track licensing via _proxy_license during download
- Apply session cookies/headers from server for CDN access
- Apply downloader/decryption config directly for remote services
- Preserve pre-injected content_keys during DASH DRM override
- Skip redundant CDM calls when all KIDs already have keys

Docs:
- Add comprehensive remote-services-flow.md with Mermaid diagrams covering proxy mode, server-CDM mode, manifest transfer, and config

unshackle/core/manifests/dash.py

@@ -305,11 +305,24 @@ class DASH:
             or (existing_drm and not any(isinstance(drm, Widevine) for drm in existing_drm))
         )
 
+        pre_existing_keys = {}
+        if existing_drm:
+            for drm_obj in existing_drm:
+                if hasattr(drm_obj, "content_keys") and drm_obj.content_keys:
+                    pre_existing_keys.update(drm_obj.content_keys)
+
         if should_override_drm:
             track.drm = manifest_drm
         else:
             track.drm = existing_drm
 
+        if pre_existing_keys and track.drm:
+            for drm_obj in track.drm:
+                if hasattr(drm_obj, "content_keys"):
+                    for kid, key in pre_existing_keys.items():
+                        if kid not in drm_obj.content_keys:
+                            drm_obj.content_keys[kid] = key
+
         manifest_base_url = manifest.findtext("BaseURL")
         if not manifest_base_url:
             manifest_base_url = track.url