Prevent runtime crashes when `releases`, `downloads`, or related lookup tables
(`releasetypes`, `schemetypes`, `sourcetypes`, `casetypes`) are absent in the
connected ZXDB MySQL database.
- Repo: gate releases/downloads queries behind a schema capability check using
`information_schema.tables`; if missing, skip queries and return empty arrays.
- Keeps entry detail page functional on legacy/minimal DB exports while fully
utilizing rich data when available.
Refs: runtime error "Table 'zxdb.releasetypes' doesn't exist"
Signed-off-by: Junie@quinn
