fix(docs): make todo-tree detect markdown checkboxes, clean up stale TASKS.md backlog

todo-tree stopped triggering on docs/TASKS.md because the file was
restructured (2026-07-28 backlog migration) to use plain markdown
checkboxes with no literal TODO/FIXME/etc. keywords, which is all
todo-tree's default config matches on. Added the documented
todo-tree.regex.regex + [ ]/[x] tags configuration (see
Gruntfuggly/todo-tree wiki) so it actually detects checkbox items, plus
red/green highlighting for open vs done.

Also cleaned up 19 stale open checkbox items left behind by that same
migration - they duplicated content already tracked as individual Gitea
issues (in old pre-migration detail, not the established "-> Issue #N"
pointer format the rest of the file already uses), including two
(Database Backup Strategy, Synapse Media PVC Backups) for issues that
are actually already closed. Converted all to pointer format or removed
where closed. Replaced the stale M1-M7 milestone table (contradicted its
own file header - said M4 "In Progress" while the summary line above
already said 0 in progress) with a pointer to the new SemVer Releases.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -50,8 +50,15 @@
           "HACK",
           "NOTE",
           "XXX",
-          "DONE"
+          "DONE",
+          "[ ]",
+          "[x]"
         ],
+        "todo-tree.regex.regex": "(//|#|<!--|;|/\\*|^|^\\s*(-|\\d+.))\\s*($TAGS)",
+        "todo-tree.highlights.customHighlight": {
+          "[ ]": { "background": "#ff000080", "icon": "issue-opened" },
+          "[x]": { "background": "#00ff0080", "icon": "check" }
+        },
         "todo-tree.tree.showScanModeButton": true,
         "todo-tree.filtering.includeGlobs": [
           "**/docs/TASKS.md",