feat: Add OpenAPI specifications for Tiingo API

Add OpenAPI 3.0.0 specifications covering Tiingo API endpoints:
- End-of-day prices
- IEX real-time data
- Crypto prices and metadata
- Forex rates
- News articles
- Fundamentals
- Mutual funds/ETFs
- Corporate actions (dividends, splits)

Specifications are modular with:
- Shared components (schemas, parameters, responses)
- Separate path files per endpoint category
- Common authentication and error handling
- Detailed request/response schemas

🤖 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'