Feature/add swagger specifics

[Lansa-18]

Closes #178

✨ Feature: OpenAPI/Swagger Specification for Indexer REST API (#178)

Goal

Add a machine-readable OpenAPI/Swagger specification to the PropChain indexer REST API so consumers can explore, test, and generate clients against the API without relying solely on the markdown README.

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🔍 What Was Implemented

The utoipa crate was integrated into the indexer to derive an OpenAPI 3.0 spec at compile time from code annotations. A Swagger UI is served at /swagger-ui/ and the raw JSON spec at /api-docs/openapi.json.

The three documented routes are:

Route	Method
/health	GET
/events	GET
/contracts	GET

Note: /metrics is intentionally excluded as it returns Prometheus text, not JSON.

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📁 Files Modified

File	Change
indexer/Cargo.toml	Added utoipa v5 and utoipa-swagger-ui v8 dependencies; declared ingest as a valid feature to fix pre-existing clippy warnings
indexer/src/db.rs	Added utoipa::ToSchema derive and doc comments to IndexedEvent
indexer/src/api.rs	Added utoipa::IntoParams + utoipa::ToSchema to EventsParams; added #[utoipa::path] annotations to health, list_events, and list_contracts handlers
indexer/src/openapi.rs	New file — ApiDoc struct that assembles the full OpenAPI spec (paths, schemas, tags, info)
indexer/src/main.rs	Added mod openapi; restructured router to merge SwaggerUi as a stateless outer layer alongside the state-bearing API routes
indexer/README.md	Added Swagger UI and OpenAPI JSON URLs under a new "API Documentation" section

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[Lansa-18]

@LaGodxy Kindly go through.