feat: input validation usage

[fredericklamar342-prog]

Description

Implements Zod-validated input schema for usage (Issue #651 — GrantFox FWC26 campaign, buffer #14).

Changes

• src/validators/usage.ts — New validator module with exported UsageQuerySchema (Zod schema) and inferred UsageQueryInput type. Schema validates:

  • from / to: ISO-8601 datetime strings (optional, with cross-field check that from <= to)
  • apiId: optional filter string
  • groupBy: "day" | "week" | "month" (optional)
  • limit: coerced integer, 1–100, default 20
  • cursor / after / before: cursor pagination tokens (optional)
  • offset: coerced non-negative integer (optional)

• src/routes/usage.ts — Removed inline UsageQuerySchema, now imports from ../validators/usage.js. Removed unused z import.

• src/validators/usage.test.ts — 30 focused unit tests covering:

  • Default values (limit defaults to 20)
  • limit bounds (0 rejected, 1–100 accepted, decimals/non-integers rejected)
  • offset validation (zero accepted, negative rejected)
  • from/to date ordering (from after to rejected)
  • groupBy enum (valid values accepted, invalid rejected)
  • apiId handling
  • cursor/after/before acceptance
  • All-fields-together smoke test
  • Empty-object defaults

Testing

• All existing usage route tests (src/routes/__tests__/usage.test.ts) pass
• New validator unit tests pass
• TypeScript compilation clean (no type errors)
• Follows project conventions: co-located validator, safeParse usage, standardized error envelope

Closes #651

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[greatest0fallt1me]

LGTM ✅ solid work — thanks for taking this on!