Follow-ups to #274: pin the clientRxCoverage.sources JSON key, warn on duplicate source names, list log limits as configurable

[dborup]

Follow-ups from the review of #274 (#265), from pve-agent1's comment on #274.

1. Test gap (trust boundary). No test loads clientRxCoverage.sources through LoadConfig, because every test builds ClientRxCoverageConfig{Sources: …} directly. A typo in the struct tag would leave the suite green while the documented key silently does nothing, so every source would be accepted. Add a LoadConfig round-trip test from JSON.
2. Hardening. Matching is case-insensitive, but nothing enforces unique mqttSources[].name. Warn at startup when one allowlisted name matches more than one configured source.
3. Docs (AGENTS rule 8). List the hardcoded 10-minute re-log interval and the 10-line cap under "Configurable values (future customizer)" in docs/client-rx-coverage.md.

[dborup]

Fixed by #290 (merged as ecff198b).