Improve README structure, accuracy, and navigation

[Copilot]

The README had grown to ~520 lines with a stale project-structure listing, finance internals ahead of the core API docs, and CORS buried inside the Streaming section. This reorganizes and corrects it; documentation-only, no source changes.

Navigation

• Added CI, Pages, Node, and license badges, plus a Contents table of contents.
• Added a Quick start block (clone → install → start → test) and merged the duplicate Installation / Running locally sections.

Accuracy

• Project structure now matches the tree: cache/, observability/, validation/, config/ (finance + cors), and all eight test files.
• Added PORT to the configuration table, with a note that the last rows (LOG_LEVEL, CORS_ALLOWED_ORIGINS, PORT) are read outside config/finance.js.
• Dropped a redundant "run the API and tests" block; rephrased "now exposes"/"Priority 1" leftovers.

Ordering

• Core API → Streaming → CORS → Excel export → Finance cluster integration → Notes.
• CORS promoted to a top-level section (it applies to /graphql, /graphql/stream, and /export, not just streaming).
• New Security and license section linking SECURITY.md and LICENSE.

API table

Grouped by operation type and cross-linked to the finance and streaming sections:

| Type | Operation | Description |
| --- | --- | --- |
| Query | `portfolioOverview(accountId, from, to, limit, offset)` | Fetch finance accounts, positions, snapshots, and P/L |
| Mutation | `createUser(name, email)` | Create a user |
| Subscription | `userCreated` | Streams every newly created user |

[Copilot]

Copilot review overview

🟢 Approval recommended

Documentation-only changes look accurate against the codebase, with only a minor Markdown formatting nit noted.

Review effort: Lite
Findings: 1

Open (1)

• Keep streaming endpoint code span on one line · New

What changed in this PR

Reorganizes and refreshes the root README to improve navigation and accuracy for the service’s core GraphQL API, SSE streaming endpoint, CORS behavior, Excel export, and the optional finance integration surface.

Changes:

• Added badges, a quick-start block, and a table of contents to make the README easier to scan.
• Updated the project structure and testing listings to match the current repository tree.
• Reordered documentation sections (API → Streaming → CORS → Export → Finance) and added a security/license section.

File	Description
README.md	Restructures and updates documentation content, navigation, and configuration details (no source changes).

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