DB Graph Importer: user tag colliding with built-in property name (scheduled) produces invalid, undeletable entity

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### Search first

- [x] I searched and no similar issues were found

### What Happened?

I imported a file graph to a DB graph ("File to DB graph") with Import all tags checked. My file graph contained a block with the inline tag `#scheduled`:
```
- DONE figure out how to schedule things for [[February 22nd, 2021]] #scheduled
```
The import completed but reported "Import detected 14 invalid block(s)... See the javascript console for more." The console shows for the `scheduled` entity:
```
:errors
{:db/ident ["should be a valid user property namespace"],
:block/tags ["should only have one tag for a built-in entity"],
:logseq.property.class/extends {"user-property" ["disallowed key"]}}
```
Entity excerpt:
```clojure
{:db/ident :user.class/scheduled-XswRbsld,
:logseq.property/built-in? true,
:logseq.property/type :datetime,
:logseq.property.class/extends #{1},
:block/name "scheduled",
:dispatch-key :property}
```
Additionally, the resulting broken tag could not be deleted from the UI, so the graph cannot be repaired after import — the only recourse was to remove `#scheduled` from the source markdown and re-import from scratch.

### Reproduce the Bug

1. Create a file graph containing a block with the inline tag `#scheduled` (lowercase; presumably any tag matching a built-in property name reproduces this)
2. Import via three dots menu → `Import` → `File to DB graph`, with Import all tags checked
3. Observe "Import detected N invalid block(s)" warning
4. Open the developer console (Ctrl+Shift+I) — the invalid entities are logged with their validation errors
5. Attempt to delete the `scheduled` tag from its page — deletion fails

### Expected Behavior

The importer should detect user tags/properties whose names collide with built-ins and either (a) merge the usage into the built-in property, (b) rename the user tag (e.g. `scheduled-user`), or (c) warn and skip — but never produce an entity that fails graph validation. Independently, invalid entities should be deletable so the graph can be repaired without a full re-import.

### Screenshots

_No response_

### Files

_No response_

### Browser, Desktop or Mobile Platform Information

Logseq 2.0.1 (Windows x64, NSIS installer), Windows 11 desktop app. File graph originally created circa 2020–2021 on older Logseq versions.

### Additional Context

Fixed at source by removing `#scheduled` from the markdown and re-importing; the re-import was clean.

### Are you willing to submit a PR? If you know how to fix the bug.

- [ ] I'm willing to submit a PR (Thank you!)

Start at the three-dots menu's Import → File to DB graph entry point and reproduce with a markdown block tagged #scheduled and Import all tags enabled. Inspect the developer-console validation errors and the tag page's deletion behavior; done means the collision no longer creates an invalid entity and the resulting graph can be repaired without re-importing.

Written by the indexing model from the issue text.