Release: bump version to 2.7.0 + default the docker publish tag to 2.7.0

- _SOULSYNC_BASE_VERSION 2.6.9 → 2.7.0 (single source of truth; drives the UI
  version + commit-suffixed build string).
- docker-publish.yml workflow_dispatch default version_tag 2.6.9 → 2.7.0 (and the
  description example), so the manual tagged-release publish defaults to 2.7.0.

.github/workflows/docker-publish.yml

@@ -9,9 +9,9 @@ on:
   workflow_dispatch:
     inputs:
       version_tag:
-        description: 'Version tag (e.g. 2.6.9)'
+        description: 'Version tag (e.g. 2.7.0)'
         required: true
-        default: '2.6.9'
+        default: '2.7.0'
 
 jobs:
   build-and-push:

web_server.py

@@ -40,7 +40,7 @@ logger = setup_logging(_log_level, _log_path)
 
 # App version — single source of truth for backup metadata, system-info, update check, etc.
 # Semver: MAJOR.MINOR.PATCH. Bump at each dev→main release.
-_SOULSYNC_BASE_VERSION = "2.6.9"
+_SOULSYNC_BASE_VERSION = "2.7.0"
 
 def _build_version_string():
     """Append short commit hash to version when available (e.g. 2.35+abc1234)."""