feat(streams): add /streams introspection endpoint for downstream WS clients

[rdemeritt]

Summary

Adds runtime introspection endpoint GET /streams for downstream WebSocket clients to query active subscriptions and client counts.

Features:

• StreamRouter.status() — pure in-memory, zero I/O, no side effects
• Compact mode (default): {"client_count": N, "subscriptions": M}
• Verbose mode (?verbose=true): includes sorted symbol lists per subscription type
• Type-switch on upstream_union (EquityStream/OptionStream) — schema documented
• Auth-gated via X-API-Key header
• 422 response on bad input (e.g., invalid verbose param)

Examples

Compact (default):

curl -H "X-API-Key: $(cat ./trader_token.json)" http://127.0.0.1:8000/streams
# {
#   "client_count": 2,
#   "subscriptions": {
#     "level_one_equity": 1,
#     "level_one_option": 1
#   }
# }

Verbose:

curl -H "X-API-Key: $(cat ./trader_token.json)" \
  'http://127.0.0.1:8000/streams?verbose=true'
# {
#   "client_count": 2,
#   "subscriptions": {
#     "level_one_equity": {
#       "count": 1,
#       "symbols": ["AAPL", "MSFT", ...]
#     },
#     "level_one_option": {
#       "count": 1,
#       "symbols": ["AAPL_061726C150", ...]
#     }
#   }
# }

Test Coverage: 30/30 QA criteria APPROVED (13/13 functional gates + 30 pytest assertions).

Files Changed

• stream_router.py — status() method + type discrimination
• main.py — GET /streams route + auth decorator
• tests/test_stream_router_status.py — new, 176 lines, 18 test cases
• tests/test_streams_endpoint.py — new, 164 lines, 12 integration tests