feat: sticky session for conversation continuity (#162 by @you922)

* feat: sticky session for multi-turn conversation continuity

Add account/sticky-session.js module that binds (callerKey, modelKey)
to a specific upstream account so multi-turn conversations stay on the
same account throughout their lifetime. Without this, the pool selects
a different account for every request and the cascade_id from the
previous turn becomes invalid — context is lost mid-task.

Integration:
- auth.js getApiKey() now accepts optional callerKey and checks
  sticky binding before falling through to normal pool selection
- chat.js handler calls setStickyBinding() after successful
  response completion, binding the caller to the account that
  just handled their last message

Configuration (opt-in):
  STICKY_SESSION_ENABLED=1        # default: 0 (off)
  STICKY_SESSION_TTL_MS=1800000   # default: 30 minutes
  STICKY_SESSION_MAX=10000        # default: 10000 max bindings

Fixes: #93, #133 (context loss mid-task)

* fix: add sticky binding in streaming response path (Oracle review)

The sticky session binding was only set in nonStreamResponse,
missing the primary streaming path used by Claude Code, Cursor,
and all OpenAI stream:true clients.

Added setStickyBinding call in streamResponse success path
(after poolCheckin, guarded by isStickyEnabled()).

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Co-authored-by: Chen Jiangjiang <chenjiangjiang@kt51.cc>