test: add table-driven tests for rental redflags

[YASSERRMD]

Adds table-driven unit tests for internal/rental/redflags, which previously had no test file.

Coverage

• TestScanIndividualRules — each rule fires in isolation (deposit wire/MoneyGram/gift-card, no-viewing, reverse-image, rent-below-market, urgency), asserting exactly one flag per isolated input and case-insensitive deposit matching.
• TestScanBoundaries — pins exact thresholds (rent ratio at 0.5 vs 0.49, urgency at 2 vs 3, zero/empty/negative inputs) to catch off-by-one regressions.
• TestScanSeverities — locks the severity (1–5) and non-empty message for every code.
• TestScanCleanSubmission — a legitimate listing produces no flags.

Verification

• gofmt clean
• go build ./... passes
• go test ./... passes
• golangci-lint run — 0 issues