feat(indexer): add Stellar RPC connectivity check to health endpoint

[sulaimonifeoluwa4-blip]

closes #105

Description

This PR adds strong, compiler-verified type safety boundaries between our asynchronous microservices. By bridging the raw string constant tuple definitions with our existing @bettapay/validation schemas via Zod inference, we eliminate type-casting gaps and introduce deterministic discriminated unions across consumer boundaries.

Changes

• 🗺️ Type Mapping Ecosystem: Exported EventPayloadMap in shared/types/index.ts dynamically leveraging z.infer to safely connect event keys to schemas.
• 🛡️ Discriminated Union: Introduced TypedEvent to ensure developers get complete autocomplete arrays and exhaustive compiler checks inside switch or conditional loops.
• 🧪 Type-Guard Utilities: Designed the isEventType predicate function enabling seamless runtime validation and type narrowing.
• 📝 Developer Guidelines: Embedded practical syntax usage code examples directly into the module export.

Acceptance Criteria Verification

• Zod Inference Integration: Mapped all operational backend event types directly to their runtime object structures.
• Type Guard Precision: The compiler narrows types flawlessly within if blocks passing the isEventType evaluation.
• Zero Build Regressions: Application modules compile without error on strict evaluation parameters.

How to Test

1. Confirm local package workspaces and declarations are successfully bound:

   npm run build