Sources that always work (botvrij, disconnect, oisd, openphish, phishtank, quad9) drop their user-facing enable_* option; the rule's enabled/disabled state is now solely controlled by the SDK rule toggle. Sources that require credentials (criminalip, malwarebazaar, otx, pulsedive, safebrowsing, threatfox, urlhaus, virustotal) instead implement the new SourcePrecheck interface so the host UI can surface "not configured" before attempting a query.
Add two shared helpers to source.go and apply them across all sources:
- disabledResult(id, name) replaces the repeated inline SourceResult literal
- evidenceEval(r, severity) replaces the identical Evaluate body in 6 sources
