fix(legal): add default user agreement + privacy policy to brand-config - #206

...onfig
The /api/settings/brand-config endpoint was returning empty legal fields
(user_agreement_url, privacy_policy_url, user_agreement_text,
privacy_policy_text all blank). This is a compliance gap for a trading
platform handling real funds.
- branding.py: add legal defaults to BRAND_DEFAULTS (summary text + URLs
 pointing to /legal/user-agreement and /legal/privacy-policy), so the
 public brand-config API returns non-empty legal metadata out of the box
- env.example: fill in the BRAND_LEGAL_* defaults so operators see the
 expected values

...wire guard into order gateway
### Platform-level risk guard (app/services/risk_guard.py)
- New RiskGuard module: 5 hard rules — drawdown halt (default 20%),
 single-symbol position ratio cap (30%), martingale layer/leverage
 lock (6 layers / 4x), order notional + ratio cap (100k / 10%),
 sliding-window rate limit (20 orders/60s)
- All thresholds configurable via RISK_GUARD_* env vars
- Thread-safe singleton (RiskGuard.shared())
- Fail-open design: if account snapshot is unavailable, guard skips
 rather than blocking trading
### Frontend error monitoring endpoint (app/routes/error_reports.py)
- POST /api/v2/errors — receives batched frontend error events
 (window.onerror, unhandledrejection, resource errors, trade/strategy
 crashes). No auth (errors can happen pre-login). Rate-limited by
 payload size cap (50 events/batch, 4k chars/field)
- GET /api/v2/errors/stats — lightweight ops counter
- Registered in openapi/register.py + tags.py as ErrorReports tag
### Risk guard integration (app/services/strategy_v2/live_execution.py)
- StrategyV2OrderGateway.submit() now calls _check_risk_guard() after
 _validate() and before persisting the pending order
- Fetches best-effort account snapshot to evaluate drawdown/position
 ratio; infers martingale state from action prefix
- Blocked orders raise ValueError('strategyV2.riskGuardBlocked: ...')
 so the caller can surface the reason to the AI strategy review
Related frontend PR: OpenByteInc/QuantDinger-Vue#8 (error_monitor.js
reports to this /api/v2/errors endpoint)