feat(backend): tip read endpoints and indexer poll loop

[manoahLinks]

Add tip read APIs and the off-chain indexer poll loop.

Tips module:

• GET /tips/:id returns a single tip (404 when missing)
• GET /profiles/:username/tips lists tips received by a profile
• GET /users/me/tips/sent lists tips sent by the authenticated user
• Cursor pagination (limit/cursor) and Zod-validated inputs; BigInt amounts serialized to decimal strings

Indexer (src/indexer):

• Poll getEvents from the stored cursor ledger forward at the configured interval, resuming via the IndexerCursor table
• Idempotent projections: EventLog deduped by (txHash, topic, ledger) and Tip upserted on the unique txHash, so replays produce no duplicates
• Wired into server bootstrap with graceful shutdown

Closes #872
Closes #873
Closes #874
Closes #893

Description

Closes #

Type of Change

Please mark the options that are relevant:

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🧪 Tests (adding new tests or updating existing tests)
• 📝 Documentation (changes to documentation/configs only)
• 🚀 DevOps & CI/CD (changes to workflows, scripts, or templates)

Changes Made

How to Test

Checklist

💻 Smart Contract Changes (if applicable)

• Running cargo fmt -- --check passes successfully.
• Running cargo clippy -- -D warnings runs without any warnings.
• All tests pass successfully using cargo test.
• New unit or integration tests have been written to cover the changes.
• No hardcoded values are present (e.g. addresses, fees) that should be configurable.

🎨 Frontend Changes (if applicable)

• TypeScript compiles cleanly with no errors (npm run typecheck or npx tsc --noEmit).
• Running npm run lint shows no linting errors.
• The production build compiles successfully via npm run build.
• Changes verified on local browser environment with Freighter/xBull/Albedo wallet.
• Responsive design verified (tested on mobile, tablet, and desktop viewport sizes).
• Keyboard navigation and accessibility (a11y) considerations are addressed.

⚙️ General

• Code follows the project's coding standards and structure guidelines.
• Self-reviewed the changes to ensure clean code with no commented-out code blocks.
• No console.log or debug code remains in production files.
• The branch is up-to-date with the main branch.

Screenshots / Demos (if applicable)

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