test: add migration engine tests

Add three tests requested in review:
- TestParseMigrationVersion: table-driven test with 9 cases covering valid
  filenames, descriptions, invalid alpha/mixed, and empty string
- TestApplyMigrations_Idempotent: verifies running migrations twice is a
  no-op with no duplicate rows in schema_migrations
- TestBootstrapMigrationsTable_FreshDatabase: isolation test verifying
  bootstrap creates schema_migrations table with version 0 row

internal/database/database_test.go

@@ -2,6 +2,7 @@ package database
 
 import (
 	"context"
+	"database/sql"
 	"fmt"
 	"path/filepath"
 	"testing"
@@ -100,3 +101,139 @@ func TestDatabaseConcurrentAccess(t *testing.T) {
 		t.Errorf("expected 10 chunks, got %d", count)
 	}
 }
+
+func TestParseMigrationVersion(t *testing.T) {
+	tests := []struct {
+		name      string
+		filename  string
+		wantVer   int
+		wantError bool
+	}{
+		{name: "valid 000.sql", filename: "000.sql", wantVer: 0, wantError: false},
+		{name: "valid 001.sql", filename: "001.sql", wantVer: 1, wantError: false},
+		{name: "valid 099.sql", filename: "099.sql", wantVer: 99, wantError: false},
+		{name: "valid with description", filename: "001_initial_schema.sql", wantVer: 1, wantError: false},
+		{name: "valid large version", filename: "123_big_migration.sql", wantVer: 123, wantError: false},
+		{name: "invalid alpha version", filename: "abc.sql", wantVer: 0, wantError: true},
+		{name: "invalid mixed chars", filename: "12a.sql", wantVer: 0, wantError: true},
+		{name: "invalid no extension", filename: "schema.sql", wantVer: 0, wantError: true},
+		{name: "empty string", filename: "", wantVer: 0, wantError: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := ParseMigrationVersion(tc.filename)
+			if tc.wantError {
+				if err == nil {
+					t.Errorf("ParseMigrationVersion(%q) = %d, nil; want error", tc.filename, got)
+				}
+				return
+			}
+			if err != nil {
+				t.Errorf("ParseMigrationVersion(%q) unexpected error: %v", tc.filename, err)
+				return
+			}
+			if got != tc.wantVer {
+				t.Errorf("ParseMigrationVersion(%q) = %d; want %d", tc.filename, got, tc.wantVer)
+			}
+		})
+	}
+}
+
+func TestApplyMigrations_Idempotent(t *testing.T) {
+	ctx := context.Background()
+
+	conn, err := sql.Open("sqlite", ":memory:?_foreign_keys=ON")
+	if err != nil {
+		t.Fatalf("failed to open database: %v", err)
+	}
+	defer func() {
+		if err := conn.Close(); err != nil {
+			t.Errorf("failed to close database: %v", err)
+		}
+	}()
+
+	conn.SetMaxOpenConns(1)
+	conn.SetMaxIdleConns(1)
+
+	// First run: apply all migrations.
+	if err := applyMigrations(ctx, conn); err != nil {
+		t.Fatalf("first applyMigrations failed: %v", err)
+	}
+
+	// Count rows in schema_migrations after first run.
+	var countBefore int
+	if err := conn.QueryRowContext(ctx, "SELECT COUNT(*) FROM schema_migrations").Scan(&countBefore); err != nil {
+		t.Fatalf("failed to count schema_migrations after first run: %v", err)
+	}
+
+	// Second run: must be a no-op.
+	if err := applyMigrations(ctx, conn); err != nil {
+		t.Fatalf("second applyMigrations failed: %v", err)
+	}
+
+	// Count rows in schema_migrations after second run — must be unchanged.
+	var countAfter int
+	if err := conn.QueryRowContext(ctx, "SELECT COUNT(*) FROM schema_migrations").Scan(&countAfter); err != nil {
+		t.Fatalf("failed to count schema_migrations after second run: %v", err)
+	}
+
+	if countBefore != countAfter {
+		t.Errorf("schema_migrations row count changed: before=%d, after=%d", countBefore, countAfter)
+	}
+}
+
+func TestBootstrapMigrationsTable_FreshDatabase(t *testing.T) {
+	ctx := context.Background()
+
+	conn, err := sql.Open("sqlite", ":memory:?_foreign_keys=ON")
+	if err != nil {
+		t.Fatalf("failed to open database: %v", err)
+	}
+	defer func() {
+		if err := conn.Close(); err != nil {
+			t.Errorf("failed to close database: %v", err)
+		}
+	}()
+
+	conn.SetMaxOpenConns(1)
+	conn.SetMaxIdleConns(1)
+
+	// Verify schema_migrations does NOT exist yet.
+	var tableBefore int
+	if err := conn.QueryRowContext(ctx,
+		"SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name='schema_migrations'",
+	).Scan(&tableBefore); err != nil {
+		t.Fatalf("failed to check for table before bootstrap: %v", err)
+	}
+	if tableBefore != 0 {
+		t.Fatal("schema_migrations table should not exist before bootstrap")
+	}
+
+	// Run bootstrap.
+	if err := bootstrapMigrationsTable(ctx, conn); err != nil {
+		t.Fatalf("bootstrapMigrationsTable failed: %v", err)
+	}
+
+	// Verify schema_migrations now exists.
+	var tableAfter int
+	if err := conn.QueryRowContext(ctx,
+		"SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name='schema_migrations'",
+	).Scan(&tableAfter); err != nil {
+		t.Fatalf("failed to check for table after bootstrap: %v", err)
+	}
+	if tableAfter != 1 {
+		t.Fatalf("schema_migrations table should exist after bootstrap, got count=%d", tableAfter)
+	}
+
+	// Verify version 0 row exists.
+	var version int
+	if err := conn.QueryRowContext(ctx,
+		"SELECT version FROM schema_migrations WHERE version = 0",
+	).Scan(&version); err != nil {
+		t.Fatalf("version 0 row not found in schema_migrations: %v", err)
+	}
+	if version != 0 {
+		t.Errorf("expected version 0, got %d", version)
+	}
+}