fix(config): use forward slash in default DB path to prevent literal backslash filename (#90)

[SAY-5]

Fixes #90. The default DefaultConnection in DeveLanCacheUI_Backend/appsettings.json:3 used backslashes:

Data Source={DeveLanCacheUIDataDirectory}\\database\\develancacheui.db

On Linux containers (the official image base) backslashes aren't path separators, so the SQLite provider created a single file literally named develancacheuidata\\database\\develancacheui.db directly inside /var, while the recommended volume mount at /var/develancacheuidata/database/ stayed empty. Container restart wiped the unmounted file → silent data loss every time.

Switched to forward slashes so the path resolves correctly under both Linux and Windows .NET (/ is accepted as a path separator by all major filesystems and SQLite providers).

Test plan

• JSON validity check on appsettings.json
• No code-side change needed; {DeveLanCacheUIDataDirectory} is the same token, only the literal separator differs