[Feature]: Validate system_config in the runtime read path

[Bccorb]

Problem statement

getSystemConfig() trusts DB rows via a cast and never parses against SystemConfigSchema; only the admin read handler validates.
There’s also a TTL comment/value mismatch (300_000 ms labeled as 30 seconds)

see code:

• src/config/getSystemConfig.ts (line 1),
• src/controllers/systemConfig.ts (line 105)

Proposed solution

Schema-validate on runtime load, fail loudly on invalid config, and align cache docs/tests

Alternatives considered

No response

Impact area

Security