docs: update references to renamed commons repository (#11)

services/docs/content/en/root/concepts/tiered-storage.md

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+---
+title: Tiered storage model
+description: Standardized storage taxonomy for service data.
+weight: 15
+---
+
+The framework enforces a standardized storage taxonomy. This ensures that every service has a predictable location for its data, which simplifies backup strategies and hardware allocation.
+
+## The storage taxonomy {#taxonomy}
+
+Data is divided into three logical [Tiers](/root/glossary#tier) based on its lifecycle, volume, and importance.
+
+### Tier 1: Configuration & critical state
+
+**Path:** `${ROOT_NODE_DIR}/@tier1`
+
+- **Contents**: Config files, small SQLite databases, SSL certificates.
+- **Characteristics**: Extremely high value, low volume.
+- **Backup strategy**: Frequent (hourly/daily), long retention.
+
+### Tier 2: Application data
+
+**Path:** `${ROOT_NODE_DIR}/@tier2`
+
+- **Contents**: User uploads, large databases, persistent application state.
+- **Characteristics**: High value, medium to high volume.
+- **Backup strategy**: Daily, prioritized for integrity.
+
+### Tier 3: Transients & bulk media
+
+**Path:** `${ROOT_NODE_DIR}/@tier3`
+
+- **Contents**: Logs, caches, temporary files, bulk media downloads.
+- **Characteristics**: Low value, high volume.
+- **Backup strategy**: Minimal or none. These directories are typically excluded from sync operations.
+
+## Justification {#justification}
+
+1. **Backup efficiency**: By separating logs (Tier 3) from configs (Tier 1), we avoid wasting storage space on redundant backups of transient data.
+2. **Hardware optimization**: Tier 1 and 2 can be placed on fast SSD storage, while Tier 3 can be mounted on slower, high-capacity HDDs.
+3. **Instant understanding**: Any operator can enter a [Node](/root/glossary#node) directory and immediately know where the critical data resides.
+
+---
+
+**See also:**
+
+- [Concept: Infrastructure architecture](/root/concepts/architecture)
+- [Reference: ROOT_ API](/root/reference/root-api)