organize deployment configs and documentation

docs/data.md

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+# Data and database compatibility
+
+ushort stores data in SQLite. The database can be mounted directly across
+upgrades; no export or import is required.
+
+```sql
+CREATE TABLE urls (
+    short_code     TEXT    PRIMARY KEY,
+    original_url   TEXT    NOT NULL,
+    created_at     INTEGER NOT NULL,
+    visit_count    INTEGER NOT NULL DEFAULT 0,
+    retention_days INTEGER NOT NULL DEFAULT 0
+);
+```
+
+Older databases missing `retention_days` are upgraded in place with a default
+of zero. Expired records are removed on startup and before each GET, POST, or
+DELETE request, using a strict expiry boundary.
+
+## Backups
+
+Take a consistent backup before every production update. SQLite's online
+backup command avoids copying a database while a WAL transaction is active:
+
+```bash
+sqlite3 data/urlshort.db ".backup data/urlshort.db.pre-update"
+sqlite3 data/urlshort.db.pre-update "PRAGMA integrity_check;"
+```
+
+Retain the backup until the updated release has passed health, redirect,
+write, and database-integrity checks.