Add database migrations for conversations and secure storage

Introduces SQL schema for persistent conversation storage and encrypted secure items, supporting the new storage architecture for managing chat history and sensitive credentials.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/owlen-core/migrations/0001_create_conversations.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS conversations (
+    id TEXT PRIMARY KEY,
+    name TEXT,
+    description TEXT,
+    model TEXT NOT NULL,
+    message_count INTEGER NOT NULL,
+    created_at INTEGER NOT NULL,
+    updated_at INTEGER NOT NULL,
+    data TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_conversations_updated_at ON conversations(updated_at DESC);

crates/owlen-core/migrations/0002_create_secure_items.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS secure_items (
+    key TEXT PRIMARY KEY,
+    nonce BLOB NOT NULL,
+    ciphertext BLOB NOT NULL,
+    created_at INTEGER NOT NULL,
+    updated_at INTEGER NOT NULL
+);