feat(server): add FLATFILES routes to the REST/WS server

[userFRM]

Problem

tools/server/src/router.rs exposes /v3/system/mdds/status and /v3/system/fpss/status but no FLATFILES routes. The startup banner only describes "MDDS gRPC for historical data" and "FPSS TCP for real-time streaming." The third surface is invisible over HTTP.

Proposed solution

Add routes under /v3/flatfiles/:

• GET /v3/system/flatfiles/status — handshake against nj-{a,b}.thetadata.us:12000, return tier/auth status.
• POST /v3/flatfiles/request — body {sec_type, req_type, date, format}. Default response: stream the decoded CSV/JSONL back to the caller (Content-Type: text/csv / application/x-ndjson). Optional ?save=path writes server-side.
• POST /v3/flatfiles/request_raw — same args, returns the raw INDEX+DATA blob with Content-Type: application/octet-stream.
• Per-sec-type/req-type convenience routes under /v3/flatfiles/option/{open_interest,trade_quote,trade,quote,eod}/{date} and /v3/flatfiles/stock/{trade_quote,trade,quote,eod}/{date}.

Add a --no-flatfiles startup flag mirroring --no-fpss so the surface can be skipped on instances without the entitlement.

Impact

HTTP / WS consumers (browser, ops scripts) gain access to the third surface. Lets the docs-site list three surfaces consistently across all bindings.

[userFRM]

Closed by PR #514. REST/WS server now exposes /v3/flatfile/... routes (commit 00acee4).