Add daily usage metrics (CPU-minutes, RAM GB-minutes)

Introduce pre-computed daily usage rollups from sandbox_metrics_snapshots.
An hourly background worker aggregates completed days, while today's
usage is computed live from snapshots at query time for freshness.

Backend: new daily_usage table, rollup worker, UsageService, and
GET /v1/capsules/usage endpoint with date range filtering (up to 92 days).

Frontend: replace Usage page placeholder with bar charts (Chart.js),
summary total cards, and preset/custom date range controls.

db/migrations/20260418072009_daily_usage.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+CREATE TABLE daily_usage (
+    team_id        UUID NOT NULL,
+    day            DATE NOT NULL,
+    cpu_minutes    NUMERIC(18, 4) NOT NULL DEFAULT 0,
+    ram_mb_minutes NUMERIC(18, 4) NOT NULL DEFAULT 0,
+    PRIMARY KEY (team_id, day)
+);
+
+-- +goose Down
+DROP TABLE daily_usage;