refactor(validation): remove unused validation middleware

[Bayovictor]

Summary

Remove dead generic schema-validation middleware (validate/createValidator) from src/middleware/validation.ts as reported in #514.

Investigation

• Grepped the entire src/ directory for createValidator, validateSchema, and middleware/validation imports — found zero production references
• Inspected all routes (admin, webhooks, roles, chains, iot, scoring-formulas, etc.) — none import or use the middleware
• Examined src/tests/validation.test.ts — it tests iotRouter and parseProjectId from errors.ts, not the deleted middleware
• The middleware's error format ({ error: "validation_error", message }) is incompatible with the established pattern ({ error: { code, message } } via ApiError/errorHandler)
• Route-local validation via badRequest()/ApiError is the clear architectural pattern in this codebase
• The middleware only validates request bodies; routes also need URL param and query param validation
• The "duplicated" validation logic (parseProjectIds, isPositiveInteger, parseProjectId) serves fundamentally different purposes

Decision

Removal was chosen because:

1. Zero production imports — genuinely dead code
2. Incompatible error format with the rest of the codebase
3. Route-local validation is the established and intentional architecture
4. The middleware provides no value that the current patterns don't already cover
5. Integrating it would require changing error formats across multiple routes (unnecessary disruption)

Changes

• Deleted src/middleware/validation.ts (122 lines)

No other files were modified. No behavior was changed.

Testing

• npm run typecheck — only pre-existing errors (unrelated to this change)
• npm run lint — only pre-existing warnings/errors (unrelated to this change)
• npm test — only pre-existing test failures (unrelated to this change)
• Confirmed zero references to deleted middleware via grep

Issue

Closes #514

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