DB: HTTP API property writes land in :plugin.property._test_plugin/*, but docs specify :plugin.property._api

Nobody has claimed this yet.

## Summary

`db-version-changes.md` states that properties created through the plugin API are namespaced to `:plugin.property._api`:

> All property related calls like `upsertBlockProperty` work with DB graphs. Property values can now be numbers and booleans. Properties are namespaced to `:plugin.property._api`

In practice, properties created through the HTTP API land in `:plugin.property._test_plugin/*`. The string `_api` does not appear anywhere in the repository.

## Cause

`src/main/logseq/api/block.cljs:28`:

```clojure
(defn get-sanitized-plugin-id
[^js plugin]
(or
(when (some-> js/window.LSPlugin (.-PluginLocal))
(some->> plugin (.-id) sanitize-user-property-name))
"_test_plugin"))
```

When there is no `PluginLocal` — the case for HTTP API callers, which are not loaded plugins — this falls back to the literal `"_test_plugin"`. That fallback is on a production path, not a test-only one: `resolve-property-prefix-for-db` uses its result to build the property namespace.

Outside this function, `_test_plugin` appears only in test files (`deps/outliner/test/logseq/outliner/op_test.cljs`, `clj-e2e/test/logseq/e2e/plugins_basic_test.clj`, and two others), which is what makes the name read as a fixture rather than a default.

## Reproduction

1. Open a DB graph in the desktop app and enable the HTTP API server.
2. Call `upsertBlockProperty` with a property name that is not yet defined, e.g. key `nsprobe`, value `probe`.
3. Query the resulting property entity:

```clojure
[:find ?ident ?type
:where
[?p :block/title "nsprobe"]
[?p :db/ident ?ident]
[?p :logseq.property/type ?type]]
```

Actual: `[":plugin.property._test_plugin/nsprobe" "default"]`
Expected per docs: `:plugin.property._api/nsprobe`

## Why it matters

Properties written through the HTTP API are permanently identified by an ident that reads as a test fixture. Idents are the stable identity for properties across graphs — they survive export and import — so this name travels with the data. If the fallback is later corrected to match the documentation, existing idents would need a migration.

## Environment

- Source verified at `master` (`abdc94bf`)
- Reproduced on a desktop build from `9a11243`, DB graph, 2026-08-14

## Possible resolutions

Either change the fallback constant to `_api` (with a migration path for data already written), or update `db-version-changes.md` to document `_test_plugin` as the actual namespace. Implementation and documentation currently disagree in either direction.

Start in src/main/logseq/api/block.cljs at get-sanitized-plugin-id and trace how resolve-property-prefix-for-db builds the property namespace. Compare that behavior with db-version-changes.md and the listed tests, then determine the appropriate implementation or documentation change and account for existing _test_plugin idents in the completion criteria.

Written by the indexing model from the issue text.