Show OpenWeather API call usage in the app

OpenWeather reports no remaining quota (only 429 when over), so add an
in-app gauge that counts the calls PilotVault itself makes and shows them
against the plan's per-minute limit.

- Plugin: process-wide usage store bucketed per API key (hashed) into the
  current minute and UTC day; a do() wrapper counts each request that
  reaches OpenWeather (transport errors consume no quota, so uncounted).
  Health and Invoke both route through it.
- New callsPerMinute config field (default 60) that cascades global -> org
  -> user; plugin contract gains plugins.HealthUsage on Health.
- Web App: a "Calls per minute limit" field and an "API call usage" meter
  in the OpenWeather card, shown after Test connection and cached.
- Counts reset on restart and cover only PilotVault's own calls (noted in
  the UI); the account dashboard stays authoritative.
- Tests for per-key counting and minute/day rollover; rebuilt frontend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API Server/internal/plugins/plugin.go

@@ -113,6 +113,18 @@ type Health struct {
 	LatencyMs int64          `json:"latencyMs,omitempty"`
 	Detail    string         `json:"detail,omitempty"`
 	Credits   *HealthCredits `json:"credits,omitempty"`
+	Usage     *HealthUsage   `json:"usage,omitempty"`
+}
+
+// HealthUsage is optional call-usage accounting a plugin may report when its
+// upstream does NOT expose remaining quota (e.g. OpenWeather). Unlike
+// HealthCredits — which reflects a balance the upstream reports — these are
+// process-local counts of the calls this server has made, bucketed into the
+// current minute and day, so the UI can render an approximate usage gauge.
+type HealthUsage struct {
+	MinuteUsed  int `json:"minuteUsed"`            // calls made in the current minute
+	MinuteLimit int `json:"minuteLimit,omitempty"` // the plan's per-minute limit
+	DayUsed     int `json:"dayUsed"`               // calls made so far today (UTC)
 }
 
 // HealthCredits is optional structured rate-limit/credit accounting a plugin may