Address market data review findings

[SahuRahul]

Implements all fixes documented in planning/MARKET_DATA_REVIEW.md.

Changes

• stream.py: create the APIRouter inside create_stream_router instead of a module-global — no shared state, no route double-registration if the factory is called twice.
• massive_client.py: pass a copy of the ticker list into the worker thread so concurrent add/remove on the event loop can't mutate it mid-request.
• massive_client.py: exponential backoff (capped at 16x) on consecutive poll failures, resetting on success — avoids hammering the API at the base interval on a sustained 429.
• massive_client.py: document the intentional add_ticker poll latency (conserves rate-limit budget).
• tests: add tests/market/test_stream.py covering the SSE endpoint.

Verification

• 77 tests pass (was 73), lint clean
• Coverage 91% -> 97%; stream.py 33% -> 97%

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