feat(ops): name 1536 MB as the supported memory floor

The getting-started requirements now say a host needs 3 GB of RAM, 4 GB
recommended, and drop the advice to lower APP_MEMORY_LIMIT to 1g on a
2 GB host. The compose and .env example comments say the same.

At boot the app reads its cgroup memory limit (v2, then v1) and logs one
line when it is below MEMORY_FLOOR_MB. The support report adds the
container limit and a note below the floor, and Admin > System status
shows "below the supported floor" next to the existing low-heap flag.
The app keeps running either way.

src/components/admin/_shared.tsx

@@ -61,8 +61,19 @@ export interface SystemStatus {
   buildTime: string;
   startTime: string;
   database: string;
-  /** The process's heap ceiling; `low` when it is below what a long-used account needs. */
-  memory: { heapLimitMb: number; low: boolean };
+  /**
+   * The process's heap ceiling; `low` when it is below what a long-used
+   * account needs. `containerLimitMb` is the container's memory limit
+   * (`null` when there is none or it cannot be read), `belowFloor` when it is
+   * under the supported floor of `floorMb`.
+   */
+  memory: {
+    heapLimitMb: number;
+    low: boolean;
+    containerLimitMb: number | null;
+    floorMb: number;
+    belowFloor: boolean;
+  };
   worker: WorkerStatus;
   /**
    * Queues whose jobs exhausted their retries recently. `null` means the

src/components/admin/system-status-section.tsx

@@ -164,11 +164,22 @@ export function SystemStatusSection() {
               icon={MemoryStick}
               label={t("admin.heapLimitLabel")}
               value={
-                status.memory.low
-                  ? t("admin.heapLimitLow", { mb: status.memory.heapLimitMb })
-                  : t("admin.heapLimitValue", { mb: status.memory.heapLimitMb })
+                status.memory.belowFloor && status.memory.containerLimitMb
+                  ? t("admin.memoryBelowFloor", {
+                      mb: status.memory.containerLimitMb,
+                      floor: status.memory.floorMb,
+                    })
+                  : status.memory.low
+                    ? t("admin.heapLimitLow", { mb: status.memory.heapLimitMb })
+                    : t("admin.heapLimitValue", {
+                        mb: status.memory.heapLimitMb,
+                      })
+              }
+              tone={
+                status.memory.belowFloor || status.memory.low
+                  ? "warning"
+                  : undefined
               }
-              tone={status.memory.low ? "warning" : undefined}
             />
             <StatusItem
               icon={Clock}