feat: Add OpenAPI foundation components

Add core OpenAPI 3.0.0 infrastructure for Tiingo API:
- Main openapi.yaml with API structure and routing
- Common schemas (Date, DateTime, Ticker, Currency, ErrorResponse)
- Reusable parameters (token, ticker, date ranges, formats)
- Standard error responses (401, 404, 500)
- Schema and component registries

This foundation enables all endpoint-specific specifications.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

openapi/responses/_index.yaml

@@ -0,0 +1,34 @@
+BadRequest: &BadRequest
+  description: Bad Request - Invalid parameters
+  content:
+    application/json:
+      schema:
+        $ref: '../schemas/common.yaml#/ErrorResponse'
+
+Unauthorized: &Unauthorized
+  description: Unauthorized - Invalid or missing API token
+  content:
+    application/json:
+      schema:
+        $ref: '../schemas/common.yaml#/ErrorResponse'
+
+NotFound: &NotFound
+  description: Not Found - Ticker or resource not found
+  content:
+    application/json:
+      schema:
+        $ref: '../schemas/common.yaml#/ErrorResponse'
+
+TooManyRequests: &TooManyRequests
+  description: Too Many Requests - Rate limit exceeded
+  content:
+    application/json:
+      schema:
+        $ref: '../schemas/common.yaml#/ErrorResponse'
+
+InternalServerError: &InternalServerError
+  description: Internal Server Error
+  content:
+    application/json:
+      schema:
+        $ref: '../schemas/common.yaml#/ErrorResponse'