Withdrawal fee calculation (2%) + realtime balance.updated event

[Godbrand0]

Summary

• Closes Withdrawals: Withdrawal fee calculation (2%) #941 — implements the 2% withdrawal fee calculation.
• Closes Realtime: Emit balance.updated events #951 — implements the balance.updated realtime event.

#941 — Withdrawal fee calculation

• Adds a calculateWithdrawalFee pure function (backend/src/modules/withdrawals/withdrawals.service.ts) that splits a gross withdrawal amount into fee and netAmount, floored so the platform never over-charges.
• Fee rate is configurable via WITHDRAWAL_FEE_BPS (env var, default 200 = 2%), wired through src/config/env.ts and src/config/index.ts, documented in .env.example.
• prepareWithdrawal now sends netAmount (post-fee) on-chain and returns fee/netAmount in the response.
• Unit tests for the fee formula (2% charge, flooring, zero-fee, invalid amount) plus an updated integration test asserting fee/netAmount on POST /withdrawals/prepare.

#951 — Realtime balance.updated event

• Adds a typed balance.updated Socket.IO server event (backend/src/realtime/types.ts) and emitBalanceUpdated (backend/src/realtime/gateway.ts), exported from the realtime barrel.
• Auth enforced the same way as notification.created: delivered only to the user:<id> room, which a socket may only join if its authenticated authUser.id matches.
• Wired into PATCH /tips/:txHash/confirm — confirming a tip changes the recipient's withdrawable balance, so their sockets are notified (best-effort; failures are logged, not surfaced as a 500 for an already-successful confirmation).
• New backend/src/realtime/gateway.test.ts covering: delivery to the balance owner after subscribing, rejection of subscribing to another user's room, and rejection of unauthenticated connections. Plus two new tests in tips.test.ts for the confirm → emit wiring.

Also fixed (blocking both issues)

The test-implement-drips branch had pre-existing merge-artifact duplication that broke npm run typecheck/test for essentially every module:

• src/app.ts — duplicated/malformed import block (syntax error) and a duplicate router mount.
• src/common/errors/AppError.ts — ServiceUnavailableError declared twice.
• prisma/schema.prisma — User model had every relation field duplicated, and TipStatus enum was declared twice (schema failed prisma generate).

These were reconciled by keeping the more complete duplicate and removing the redundant one — no behavioral changes beyond making the branch buildable/testable again.

Test plan

• npm run typecheck — no new errors (pre-existing unrelated errors in credit.service.ts / x.test.ts remain, not touched by this PR)
• npm run lint — 0 errors (pre-existing warnings only)
• npx vitest run src/modules/withdrawals/withdrawals.test.ts — 9/9 passing
• npx vitest run src/realtime/gateway.test.ts — 3/3 passing
• npx vitest run src/modules/tips/tips.test.ts -t confirm — 7/7 passing (includes 2 new balance.updated tests)
• Full suite has pre-existing failures unrelated to this change (tests requiring a live Postgres/Redis via docker compose up, and unrelated bugs in credit, x, auth modules) — not introduced by this PR.

[Akanimoh12]

Please resolve conflicts

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