feat: v0.2.0 branding, healthz version, SSE dashboard live

Centralize APP_VERSION 0.2.0; /health and /healthz return version;
startup log line; UI title Security Alert Center v.0.2.0;
SSE /api/v1/stream/events for live dashboard counters.

backend/app/api/v1/stream.py

@@ -0,0 +1,62 @@
+import asyncio
+import json
+from datetime import datetime, timedelta, timezone
+
+from fastapi import APIRouter, Depends, Query
+from fastapi.responses import StreamingResponse
+from sqlalchemy import func, select
+from sqlalchemy.orm import Session
+
+from app.auth.jwt_auth import verify_access_token
+from app.database import SessionLocal
+from app.models import Event, Problem
+from app.version import APP_VERSION
+
+router = APIRouter(prefix="/stream", tags=["stream"])
+
+SSE_INTERVAL_SEC = 5
+
+
+def _dashboard_snapshot(db: Session) -> dict:
+    since = datetime.now(timezone.utc) - timedelta(hours=24)
+    events_24h = db.scalar(
+        select(func.count()).select_from(Event).where(Event.received_at >= since)
+    ) or 0
+    problems_open = (
+        db.scalar(select(func.count()).select_from(Problem).where(Problem.status == "open")) or 0
+    )
+    last_event = db.scalar(select(Event.id).order_by(Event.received_at.desc()).limit(1))
+    return {
+        "type": "dashboard",
+        "version": APP_VERSION,
+        "events_last_24h": events_24h,
+        "problems_open": problems_open,
+        "last_event_id": last_event,
+        "at": datetime.now(timezone.utc).isoformat(),
+    }
+
+
+async def _sse_generator():
+    while True:
+        db = SessionLocal()
+        try:
+            payload = _dashboard_snapshot(db)
+        finally:
+            db.close()
+        yield f"data: {json.dumps(payload, ensure_ascii=False)}\n\n"
+        await asyncio.sleep(SSE_INTERVAL_SEC)
+
+
+def _sse_auth(token: str = Query(..., description="JWT access_token")) -> str:
+    return verify_access_token(token)
+
+
+@router.get("/events")
+async def stream_events(
+    _user: str = Depends(_sse_auth),
+) -> StreamingResponse:
+    return StreamingResponse(
+        _sse_generator(),
+        media_type="text/event-stream",
+        headers={"Cache-Control": "no-cache", "X-Accel-Buffering": "no"},
+    )