Startup scan: warn (don't silently skip) when the reserved `restore` metadata dir looks like a database

[cb1kenobi]

Follow-up from #1831 review (flagged by @kriszyp on resources/databases.ts).

Context

#1831 stores restore lock/marker metadata in a reserved restore directory beside the database directories. The backtick makes it an illegal database name (schemaRegex rejects / and backtick), so the API can never create a database that collides with it, and the startup scan explicitly skips that entry.

Edge case

The startup scan opens any directory containing CURRENT + MANIFEST-* without re-applying schemaRegex. So if a directory literally named restore already exists on disk as a RocksDB database (only reachable via manual filesystem creation, since the API always rejected backticks), an upgrade to this code now silently stops loading that database, and a subsequent restore would write .lock/.restoring metadata into it.

Severity

Very low probability (no API path can produce such a directory), but the silent disappearance is a bad state to hit undiagnosed.

Suggested fix

When the scan skips the reserved restore entry, detect whether it looks like a real database (has CURRENT + MANIFEST-*) and log a warning (or refuse startup with a clear message) rather than silently ignoring it, so an operator can migrate/rename it.

Ref: #1831, resources/databases.ts scan loops.