feat: add Network Backup section to Database Management

PC2 can now automatically pull and restore a fresh copy of PC1's database
on a daily schedule. Config and API key are stored in local JSON files so
they survive database restores.

- New networkSyncConfigService: file-based config (network-backup-key.json,
  network-sync-config.json) that persists through DB restores
- New networkSyncService: streams live pg_dump from source PC over HTTP and
  pipes into psql, then reconnects Prisma and applies missing migrations
- 6 new endpoints: get/regenerate API key, serve backup stream (key-auth
  only), get/save sync config, trigger immediate sync
- Hourly cron job that fires only when current hour matches configured syncHour
- NetworkBackupManager component: shows this machine's key (show/copy/regen)
  and receiver config (enable toggle, hour picker 0-23, source URL + key,
  Save + Sync Now, last sync status)
- README setup guide for both PCs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -450,6 +450,38 @@ Paste this key into the Activation page in the app.
 
 ---
 
+## Network Backup Setup (PC-to-PC Sync)
+
+Two PCs running the app can be linked so the backup PC automatically pulls a fresh copy of the main PC's database every night. The config survives database restores because it is stored in local files, not in the database.
+
+**Prerequisites:** Both PCs must be on the same local network (e.g. connected to the same router or switch). Set a static IP on the main PC so its address never changes after a reboot (set in the OS network settings, not in the router).
+
+### On PC1 (main server)
+
+1. Open the app → **Database Management** → **Network Backup**
+2. Under **This Machine's Backup Key**, click the eye icon to reveal the key
+3. Click the copy button to copy it
+
+### On PC2 (backup PC)
+
+1. Open the app → **Database Management** → **Network Backup**
+2. Under **Sync from Another PC**:
+   - Toggle **Enable daily sync** on
+   - Select the hour you want the sync to run (e.g. `12:00 AM (midnight)`)
+   - Enter PC1's URL in the **Source PC URL** field, e.g. `http://192.168.0.94:3000`
+   - Paste PC1's key into the **Source PC API Key** field
+3. Click **Save Settings**
+4. Click **Sync Now** to test — PC2's database will be replaced with PC1's
+
+After a successful test, the sync will run automatically at the scheduled hour every day.
+
+**Notes:**
+- The API key and sync config are stored in `apps/Backend/network-backup-key.json` and `apps/Backend/network-sync-config.json` — they survive database restores
+- If you regenerate PC1's key, you must update it on PC2 as well
+- The sync is one-way: PC2 always mirrors PC1; PC1 is never modified
+
+---
+
 ## Claude Code Memory
 
 Claude Code (the AI assistant used to build this project) stores its memory locally on the PC. This memory contains project context, architecture decisions, feature history, and working preferences — allowing Claude to pick up where it left off in new sessions.