feat: Add recent performance stability stats (matches/days) to player profile

ETL/README.md

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+# ETL Pipeline Documentation
+
+## 1. L1A (Raw Data Ingestion)
+**Status**: ✅ Supports Incremental Update
+
+This script ingests raw JSON files from `output_arena/` into `database/L1A/L1A.sqlite`.
+
+### Usage
+```bash
+# Standard Run (Incremental)
+# Only processes new files that are not yet in the database.
+python ETL/L1A.py
+
+# Force Refresh
+# Reprocesses ALL files, overwriting existing records.
+python ETL/L1A.py --force
+```
+
+L1B demoparser2 -> L1B.sqlite
+
+L2 L1A.sqlite (+L1b.sqlite) -> L2.sqlite
+
+L3 Deep Dive