docs(observability): document server.observability.usage_audit in config hierarchy

[r266-tech]

The Observability config hierarchy section lists server.observability.{metrics,traces,logs,dump_body} but omits server.observability.usage_audit, even though it is a real wired subkey of ObservabilityConfig.

Source-grounded in openviking/server/config.py:

• class UsageAuditConfig (L89): enabled: bool = True (L92), backend: Literal["sqlite"] = "sqlite" (L93), sqlite_path: Optional[str] = None (L94), timezone: str = "local" (L102)
• wired via class ObservabilityConfig → usage_audit: UsageAuditConfig = Field(default_factory=UsageAuditConfig) (L126)

This gap became conspicuous after #2442: the multi-instance deployment notes (03-deployment.md) now direct operators to set server.observability.usage_audit.sqlite_path for per-instance paths, but the observability config hierarchy — the canonical place that enumerates the server.observability.* subkeys — does not list usage_audit at all, so an operator cannot discover enabled/sqlite_path/timezone from the hierarchy.

Adds the missing usage_audit bullet + an entry in the config example, matching the existing dump_body documentation style (#2063). EN + ZH mirrored.

[github-actions]

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🎫 Ticket compliance analysis ✅ 2442 - Fully compliant Compliant requirements: Add server.observability.usage_audit to the observability config hierarchy in docs Update both English and Chinese documentation Include usage_audit in the config example Match existing documentation style

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

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