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/parameters/_index.yaml

@@ -0,0 +1,49 @@
+# Reusable Parameter Definitions for Tiingo API
+# Reference these using: $ref: '../parameters/_index.yaml#/ParameterName'
+
+TokenParam: &TokenParam
+  name: token
+  in: query
+  required: true
+  schema:
+    type: string
+  description: API token for authentication
+
+TickerPathParam: &TickerPathParam
+  name: ticker
+  in: path
+  required: true
+  schema:
+    type: string
+    pattern: '^[A-Z0-9]{1,5}$'
+  description: Stock or cryptocurrency ticker symbol
+
+StartDateParam: &StartDateParam
+  name: startDate
+  in: query
+  required: false
+  schema:
+    type: string
+    format: date
+  description: Start date for historical data in YYYY-MM-DD format
+
+EndDateParam: &EndDateParam
+  name: endDate
+  in: query
+  required: false
+  schema:
+    type: string
+    format: date
+  description: End date for historical data in YYYY-MM-DD format
+
+FormatParam: &FormatParam
+  name: format
+  in: query
+  required: false
+  schema:
+    type: string
+    enum:
+      - json
+      - csv
+    default: json
+  description: Response format (json or csv)