Add database module with connection manager and schema initialization

- Introduced `DatabaseManager` for handling SQLite connections per user with WAL and foreign key support.
- Implemented `initialize_schema` to set up comprehensive database schema covering AI, books, characters, locations, series, and sync tracking.
- Added migration helpers for schema versioning and column updates.
- Structured database module into `connection.rs` and `schema.rs` for clearer organization.

src-tauri/src/db/connection.rs

@@ -0,0 +1,62 @@
+use rusqlite::Connection;
+use std::collections::HashMap;
+use std::path::PathBuf;
+use std::sync::{Arc, Mutex};
+
+use crate::error::{AppError, AppResult};
+
+pub struct DatabaseManager {
+    connections: HashMap<String, Connection>,
+    base_path: PathBuf,
+}
+
+impl DatabaseManager {
+    pub fn new(base_path: PathBuf) -> Self {
+        Self {
+            connections: HashMap::new(),
+            base_path,
+        }
+    }
+
+    pub fn get_db_path(&self, user_id: &str) -> PathBuf {
+        self.base_path.join(format!("eritors-local-{}.db", user_id))
+    }
+
+    pub fn initialize(&mut self, user_id: &str) -> AppResult<()> {
+        if self.connections.contains_key(user_id) {
+            return Ok(());
+        }
+
+        let db_path = self.get_db_path(user_id);
+
+        if let Some(parent) = db_path.parent() {
+            std::fs::create_dir_all(parent)?;
+        }
+
+        let conn = Connection::open(&db_path)?;
+
+        conn.execute_batch("PRAGMA journal_mode=WAL;")?;
+        conn.execute_batch("PRAGMA foreign_keys=ON;")?;
+
+        self.connections.insert(user_id.to_string(), conn);
+        Ok(())
+    }
+
+    pub fn get_connection(&self, user_id: &str) -> AppResult<&Connection> {
+        self.connections
+            .get(user_id)
+            .ok_or_else(|| AppError::Database(rusqlite::Error::InvalidParameterName(
+                format!("No database connection for user: {}", user_id),
+            )))
+    }
+
+    pub fn close(&mut self, user_id: &str) {
+        self.connections.remove(user_id);
+    }
+}
+
+pub type DbManager = Arc<Mutex<DatabaseManager>>;
+
+pub fn create_db_manager(base_path: PathBuf) -> DbManager {
+    Arc::new(Mutex::new(DatabaseManager::new(base_path)))
+}