refactor: move workers/ into apps/workers/; add backend/README.md

[Rakesh1002]

Summary

Continues the directory reorg. Adopts a single rule: every deployable TypeScript / SDK package lives under apps/*. The Python backend stays at backend/ because it's Python (different language, conventionally one source tree per project, and its app/ module would collide with apps/ if nested).

Before:
```
unsearch/
├── backend/ Python
├── workers/ TypeScript (CF Workers) ← inconsistent shelf
└── apps/
├── web/ TypeScript (CF Workers)
└── sdk-*/
```

After:
```
unsearch/
├── backend/ Python (the only non-apps/ deployable)
└── apps/
├── workers/ TypeScript (CF Workers edge router) — moved
├── web/ TypeScript (CF Workers — Next.js dashboard)
├── sdk-ts/, sdk-py/, sdk-llamaindex/, mcp-server/ (planned)
```

Mechanical changes

• git mv workers apps/workers — every TS source file moved with history preserved.
• pnpm-workspace.yaml: collapses to a single - "apps/*" glob (drops the now-redundant - "workers" line).
• apps/workers/containers.toml: build context .. → ../..; dockerfile path ../backend/Dockerfile.cloudflare → ../../backend/Dockerfile.cloudflare.
• .github/workflows/deploy-cf.yml: every working-directory: workers, workingDirectory: workers, and workers/pnpm-lock.yaml reference → apps/workers.
• scripts/cf-provision.sh: cd into apps/workers; output paths updated.
• backend/Dockerfile.cloudflare: header comment references apps/workers/containers.toml.
• All docs: workers/... → apps/workers/... in docs/architecture.md, docs/what-is-what.md, docs/README.md, docs/citation-envelope.md, docs/roadmap.md, docs/strategy/user-journey.md, docs/strategy/pricing.md, ADR-0001, ADR-0010, and the README.md repo tree.
• ADR-0006 Amendment Part 3: documents this move + the new "single apps/*" rule.

What did NOT change

• The Worker's internal Hono routes, Durable Object bindings, D1 schema, MCP transport, wrangler.toml, and all source code are unchanged — only their file system location.
• No runtime behaviour, no public API surface, no env vars, no wire formats.
• CHANGELOG.md intentionally untouched — historical release notes refer to the workers/ path that was correct at the time; rewriting history is wrong.

Other audit cleanup in this PR

apps/backend/ directory removed

The empty apps/backend/ directory left behind by the previous restructure PR (it contained only a gitignored .env) is gone. The .env was filesystem-copied to backend/.env in this workspace, but since both .env files are gitignored neither is in the diff. Users with their own local checkouts will need to do the same dance:

```bash
cp apps/backend/.env backend/.env
rm -rf apps/backend
```

Added backend/README.md

Every other package (apps/web, apps/workers, apps/sdk-ts, apps/sdk-py, apps/sdk-llamaindex) had a per-package README. backend/ was the lone exception. Added one — covers layout, quickstart (make dev, make test, etc.), env setup, request flow from the Worker, citation envelope pointer.

Risk

• CI: all apps/workers paths in deploy-cf.yml updated; expect green.
• Production deploy: unchanged. Worker code unchanged. Container image unchanged.
• Self-host: unchanged. docker-compose paths weren't affected by this PR.
• Local dev: anyone with a checked-out branch needs to delete their root workers/ checkout and pull fresh (or git will refuse to clean a renamed-away dir).

Test plan

• `git pull origin main && ls` — no top-level workers/; apps/workers/ exists.
• `ls apps/backend/` returns no such file or directory.
• `ls backend/` includes README.md, app/, alembic/, tests/, etc.
• `pnpm install` at root resolves apps/workers as a workspace member.
• CI green on deploy-cf.yml worker + container + web + smoke jobs.
• `wrangler containers deploy --config apps/workers/containers.toml` finds ../../backend/Dockerfile.cloudflare.
• `cd apps/workers && pnpm test` still runs the vitest suite.