docs: add v1 to v2 migration guide

Documents breaking changes including the need to re-add DM contacts
to the device firmware table after migration.

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

docs/MIGRATION-v1-to-v2.md

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+# Migration Guide: v1 to v2
+
+## Overview
+
+v2 replaces the `meshcore-cli` bridge with direct `meshcore` library communication. This changes how contacts and DMs work at a fundamental level.
+
+## Breaking Changes
+
+### 1. DM contacts must be re-added to the device
+
+**Symptom**: Sending a DM to an existing contact fails with "Contact not on device".
+
+**Why**: In v1, `meshcore-cli` acted as a bridge and managed contacts independently. In v2, `meshcore` communicates directly with the device firmware, which **requires** the contact to exist in its internal contact table (max 350 entries) to send a DM. After migration, the database has contacts from v1 advert history, but most of them were never added to the device's firmware contact table.
+
+**Fix**: For each contact you want to DM:
+1. Delete the contact from the Contacts page
+2. Wait for their next advertisement
+3. Approve the contact when it appears in the pending list
+
+This adds the contact to the device's firmware table, enabling DM sending.
+
+**Note**: Incoming DMs from any contact still work regardless — this only affects *sending* DMs.
+
+### 2. Contact soft-delete preserves DM history
+
+In v2, deleting a contact is a soft-delete (marked as `source='deleted'` in the database). This preserves DM conversation history. When the contact is re-added, it automatically "undeletes" and all previous DMs are visible again.
+
+In v1, deleting a contact would orphan DM records (set `contact_pubkey = NULL`), causing "Unknown" entries in the DM list.
+
+### 3. Database schema
+
+v2 uses SQLite with WAL mode instead of flat JSON files. The migration from v1 data happens automatically on first startup (see `app/migrate_v1.py`). The v1 data files are preserved and not modified.
+
+## Post-Migration Checklist
+
+- [ ] Verify device connection (green "Connected" indicator)
+- [ ] Check that channel messages are flowing normally
+- [ ] For each DM contact you need: delete, wait for advert, re-approve
+- [ ] Verify DM sending works with a test message