Add stream POST negative tests

[Emelie-Dev]

closes #815

[ogazboiz]

your branch is green, but it went conflicting after the ci fixes landed on main: #969 and #974 touched backend/Dockerfile and backend/src/workers/soroban-event-worker.ts, which this pr also edits. please rebase on main and it should be good to go: git fetch origin && git rebase origin/main && git push --force-with-lease. if any of your Dockerfile or soroban-event-worker changes overlap what #969 already did (copying prisma/ into the runner, and the import type { Prisma } from '../generated/prisma' fix), drop those now-redundant bits and keep just your actual change. i'll re-review and merge once it's rebased and ci is green.

[ogazboiz]

the negative tests themselves are good (400 on malformed/missing numeric fields, 403 on non-owner reactivation, asserting prisma.stream.upsert isn't called). but this branch is based on stale main and would regress it, so it needs a rebase before it can land:

1. it reverts #969's fix, soroban-event-worker.ts changes import { prisma, Prisma } back to import { prisma } (dropping the Prisma import), whereas current main uses import type { Prisma } from '../generated/prisma'. merging this would re-break the typecheck fix #969 landed.
2. stale ci.yml, it removes the "Upload frontend coverage to Codecov" step that main currently has.
3. it rewrites .github/dependabot.yml (collapsing the per-dir entries into one root) and mass-reformats several backend files (single->double quote churn), all unrelated to negative tests and overlapping earlier dependabot work.

please rebase on current main and drop everything except the actual negative tests, then it's a clean add. as-is it would revert the ci fixes.

[ogazboiz]

good progress, and thanks for addressing the earlier regressions: the Prisma import is back to ../generated/prisma (correct), and the ci.yml and dependabot.yml changes are gone. the negative tests themselves are solid coverage. but it's still carrying unrelated changes under a "negative tests" title that need to come out before it can land:

1. it rewrites backend/Dockerfile, changing the COPY prisma.config.ts ./ line (which the restored Backend Docker Image CI relies on) to COPY --from=builder /app/prisma.config.ts. even though ci is green, that's a modification to infra i recently fixed and it doesn't belong in a tests pr. please drop it.
2. app.ts has pure reformatting churn (multi-line import rewrap, .map/.filter re-indent) unrelated to the tests. drop that too.

trim it down to the actual negative tests (plus only whatever minimal change is genuinely needed for them to run) and it's a clean, welcome add. right now +1383/-554 for "negative tests" is mostly churn, and the Dockerfile rewrite is the part i really don't want slipping in unnoticed.