fix: validate limit/offset on webhook management routes with OffsetPaginationSchema

[aojomo]

Summary

Add offset-based pagination validation to webhook management routes (GET /deliveries and GET /dlq) using a new OffsetPaginationSchema in the shared pagination validation module. Non-numeric, negative, zero, and excessively large limit/offset values now return a clear 400 INVALID_PAGINATION error instead of silently producing empty or wrong-sized pages.

Fixes Fluxora-Org#847

Changes

src/validation/paginationSchema.ts

• Added OffsetPaginationSchema — a Zod schema for offset-based pagination with limit (1-100) and offset (>= 0) fields, both optional to allow route-specific defaults. Reuses the existing MAX_PAGE_LIMIT (100) constant for consistency with PaginationSchema and streamRepository.MAX_PAGE_SIZE.

src/routes/webhooks.ts

• GET /deliveries: Validates limit and offset via OffsetPaginationSchema. Rejects invalid values with 400. Default limit: 100, default offset: 0.
• GET /dlq: Validates limit via OffsetPaginationSchema. Rejects invalid values with 400. Default limit: 50.

tests/webhooks-pagination.test.ts (new)

21 tests covering:

• Non-numeric limit/offset -> 400
• Negative limit/offset -> 400
• Zero limit -> 400
• Decimal (non-integer) limit -> 400
• Limit exceeding MAX_PAGE_LIMIT (100) -> 400
• Valid values -> 200
• Defaults when omitted
• Offset beyond total -> empty array

Acceptance Criteria

• Non-numeric limit/offset values return a clear 400 instead of silently empty/wrong page
• A maximum page size is enforced server-side regardless of caller input
• Tests cover the identified edge cases for both routes
• Reuses existing pagination validation infrastructure from paginationSchema.ts
• Negative values reject with clear 400
• All 21 new tests pass (pre-existing failures are unrelated metrics counter issues)