chore(tsconfig): enable stricter TypeScript compiler checks

[MerlinTheWhiz]

Closes #652

Changes made:

backend/tsconfig.json — Uncommented noImplicitReturns, noUnusedLocals, noFallthroughCasesInSwitch

Fixes for noImplicitReturns (TS7030):

• src/app.ts:105 — added return before next()
• src/controllers/sse.controller.ts:83 — added return after sseService.addClient()
• src/controllers/user.controller.ts:33,64,122,163 — changed next(error) to return next(error) in all 4 catch blocks
• src/middleware/error.middleware.ts:52 — added return before res.status(...).json(...)

Fixes for noUnusedLocals (TS6133):

• src/controllers/stream.controller.ts:283 — removed dead direction variable
• src/lib/prisma-sandbox.ts — removed unused databaseUrl variable, config variable, and getSandboxConfig import
• src/services/sorobanService.ts:1 — removed unused Account import
• src/services/sorobanService.ts:209,243 — removed unused retval assignments
• tests/integration/top-up.test.ts:80 — removed unused prisma import
• tests/rate-limiter.test.ts:1 — removed unused beforeEach import
• tests/sandbox.middleware.test.ts:2 — removed unused isSandboxRequest import
• tests/soroban-indexer.test.ts:1 — removed unused expect import
• tests/soroban-worker.helpers.test.ts:1 — removed unused vi import
• tests/soroban.service.test.ts:1 — removed unused vi, beforeEach imports
• tests/sse.service.test.ts:3 — removed unused sseService import
• tests/stream-rate-limiter.test.ts:1 — removed unused vi, afterEach imports; removed unused limiter variables (lines 249, 265)
• tests/user-summary-cache.test.ts:26 — removed unused TTL_MS constant

Fixes for TS6196 (unused declaration):

• src/services/claimable.service.ts:28 — removed unused ClaimableCacheEntry interface

npm run build passes clean. There were 10 test failures in stream-lifecycle.test.ts and they are pre-existing DB auth issues, not related to these changes (199 tests pass).

[ogazboiz]

clean — closes #652 (backend tsconfig enables noImplicitReturns / noUnusedLocals / noUnusedParameters across 19 files). all CI green. merging!

join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0

[ogazboiz]

approved! but a couple of conflicting merges landed first and now this branch is DIRTY. quick rebase and it can land:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

i'll auto-merge as soon as it's clean.

[MerlinTheWhiz]

@ogazboiz Done boss. Followed your instruction cleanly, please review then merge

[ogazboiz]

clean — closes #652, stricter tsconfig (noImplicitReturns / noUnusedLocals / noUnusedParameters) enabled across 19 backend files. all CI green. merging!

join us on Telegram: https://t.me/+DOylgFv1jyJlNzM0