feat(backend): replace WebSocket payment updates with SSE

[Bayovictor]

Closes #730

What

Replaces the Socket.io/WebSocket realtime channel with plain-HTTP Server-Sent Events for one-way payment status updates: one socket plus one map entry per client instead of a WS upgrade, frame protocol and server-side ping bookkeeping.

• stellar-payment-platform/src/sse/index.js — client registry keyed by payment topic, 25s keep-alive heartbeats, retry: hint, in-process emitPaymentUpdate, Redis publishPaymentUpdate for cross-process fan-out, and closeSse() drain used by graceful shutdown (keeps the legacy wire channel and accepts the old paymentIntentId field)
• GET /api/v1/payments/:paymentId/events (src/routes/v1/sseRoutes.js) — emits connected, a snapshot of the last known status (so an EventSource reconnect converges immediately) and payment-update on every change; headers are flushed before any async work so connect-timeout/compression cannot buffer or abort the stream
• SSE paths are exempt from the global token bucket and bounded instead by a dedicated per-IP stream limiter
• horizonListener.js now publishes detected payments to Redis (the producer was imported but never called), so the pipeline is live end to end
• Dropped socket.io and src/websocket; nginx gets a no-buffer/no-timeout proxy location for the stream; .env.example, EVENT_SOURCING.md and README.md updated
• payment-dashboard/src/lib/paymentEvents.js wraps EventSource, so reconnection is handled natively by the browser (AC)
• Fixed the broken lint-staged entry so dashboard files lint from the repo root

Acceptance criteria

• SSE endpoints stream status updates to clients — GET /api/v1/payments/:paymentId/events
• Reconnection logic handled natively by the EventSource API — payment-dashboard/src/lib/paymentEvents.js
• Server resource consumption drops vs WS — Socket.io and src/websocket removed; per client cost is a single HTTP stream + map entry

Verification

• npx jest tests/sse.test.js tests/horizon-listener.test.js — 28 passed (manager, route, Redis fan-out, shutdown, listener regression)
• npx jest server.test.js — 67 passed
• npx eslint on the touched files — 0 errors (2 pre-existing no-unused-vars warnings on horizonListener.js, present on main)

Related: #859

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