Rewrite README to match the actual code

[sultanmaliki]

Summary

README.md was the only markdown file in the repo, and it had drifted from the implementation. This rewrites it against the current backend and frontend source. It is a docs-only change.

What was wrong and is now fixed

• API docs: POST /api/query request/response shapes were wrong (it does not accept sourceType/connectionString and does not return usage/raw). /api/query/demo, the health check, both rate limiters, upload/execute payloads (UUID file ids, imported_csv/imported_json tables, structured mongo payload, Neo4j credentials) are now documented as implemented.
• Supported databases: separates languages that are only generated (CQL, Redis, Elasticsearch, DynamoDB, GraphQL) from those that can be executed (SQLite/CSV/JSON/SQL files, PostgreSQL, MySQL/MariaDB, MongoDB find, Neo4j).
• Configuration: lists only the env vars the code reads; removes TOKEN_EXPIRY and TEST_*, which are never used. Notes that there is no .env.example.
• Models: adds the alias table and the Auto routing heuristics.
• Conversation memory: chat context is the last three completed exchanges; utils/conversationMemory.js is not called by any route.
• Security: adds notes on the unauthenticated /api/db/* routes, open CORS, the default JWT secret and the unused xss dependency. The old README both claimed CORS was restricted and said it was open.
• Other: Windows note for the POSIX-style npm scripts, Docker caveats, corrected project structure/tech stack/contributing, and a Mermaid architecture diagram in place of the misaligned ASCII one (rendered and checked).

Not changed (noticed while reviewing, out of scope)

• .github/workflows/node.js.yml runs npm ci/npm test at the repo root, but there is no root package.json (and package-lock.json is git-ignored), so the workflow cannot pass as written.
• /api/db/* has no auth middleware, and utils/conversationMemory.js is unused.

Test plan

• Every claim cross-checked against querycraft-backend/ and querycraft-frontend/src/
• Mermaid diagram renders without errors
• Internal anchor links match the headings

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