feat!: API keys actually gate the gateway — auth, attribution, rate limits, budgets

The office-readiness audit found the foundation missing: configured API keys
were decorative. Any bearer — or none — passed every model endpoint, per-key
rate limits existed only in the form, and all real traffic was recorded as
"anonymous", leaving the per-key spend table permanently blank.

A middleware on the model surface (/v1/*, /route, /mcp) now enforces keys:
- no keys configured → open mode as before (the console already warns);
- keys exist → Bearer or x-api-key must match (401 invalid_api_key);
- per-key rate_per_min → fixed one-minute window, 429 with Retry-After;
- NEW per-key monthly_budget_usd → 429 insufficient_quota once the account's
  month-to-date spend crosses the ceiling;
- the matched account rides the request into stats — the keys table's spend
  columns finally see real traffic.

Month-to-date spend is tracked per account in stats and persisted in the
totals sidecar with a watermark (no double counting across restarts). The
key dialog gains a budget field; the table shows "budget $N/mo" in red once
exhausted. All verified live: 401 wrong/missing key, 200 + attribution with
the right key, 429 + Retry-After over the limit, 429 insufficient_quota on
the second request past a $0.000001 budget.

Also: the dashboard shows the OpenRouter credit balance (GET
/api/v1/credits via the stored key, cached 5 min, hidden without a key, red
under $5 — live: $16.18), the server drains connections on Ctrl-C instead
of dropping them, request bodies are capped at 32 MB, and docs/DEPLOY(.ru)
cover office deployment: systemd/launchd/Windows service, nginx/caddy TLS,
security checklist, backups. 10 strings ×7 languages.

src/config.rs

@@ -620,6 +620,9 @@ pub struct ApiKeyCfg {
     pub account: String,
     #[serde(default)]
     pub rate_per_min: u32,
+    /// Monthly spend ceiling in USD for this key's account; 0/absent = none.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub monthly_budget_usd: Option<f64>,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub allow_models: Vec<String>,
 }