feat(receiver): storage layer and webhook payload compat

Dual-dialect store (SQLite via modernc.org, MySQL via go-sql-driver,
both pure Go) with order-tolerant start/complete upserts, filtered and
paginated listing, and aggregate queries (per-day, top domains, query
types, statuses, duration percentiles, top clients). Sender gains
optional EXPLOREDNS_WEBHOOK_TOKEN bearer auth; a round-trip test pins
receiver structs byte-compatible with the sender payloads.

Note: go directive moves to 1.25.0, required by modernc.org/sqlite.
CI reads the version from go.mod so GOTOOLCHAIN=local stays satisfied.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/receiver/store/dialect.go

@@ -0,0 +1,103 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"strings"
+)
+
+// dialect abstracts the SQL syntax differences between the sqlite and
+// mysql backends. Both drivers use '?' placeholders, and all datetimes are
+// bound and scanned as "2006-01-02 15:04:05.000000" UTC strings so value
+// handling stays identical; only the constructs below diverge.
+type dialect interface {
+	name() string
+
+	// datetimeType is the column type for datetime values. sqlite has no
+	// datetime type (TEXT affinity stores our formatted strings verbatim,
+	// which sort and compare lexically); mysql needs DATETIME(6) because
+	// plain DATETIME truncates the sub-second precision we write.
+	datetimeType() string
+
+	// upsert appends the insert-or-update clause to an INSERT statement,
+	// overwriting cols with the values from the attempted insert. sqlite
+	// spells this ON CONFLICT(id) DO UPDATE SET col=excluded.col; mysql
+	// spells it ON DUPLICATE KEY UPDATE col=VALUES(col) (VALUES() is
+	// deprecated in MySQL 8.0.20+ but is the only form MariaDB and older
+	// MySQL also accept).
+	upsert(insert string, cols []string) string
+
+	// createIndex creates an index if it does not exist. sqlite supports
+	// CREATE INDEX IF NOT EXISTS; mysql has no IF NOT EXISTS for indexes,
+	// so existence is checked via information_schema first.
+	createIndex(ctx context.Context, db *sql.DB, index, table, column string) error
+
+	// likeContains returns a "col contains substr" predicate plus its
+	// bind argument, with LIKE wildcards in substr escaped. The ESCAPE
+	// literal differs: mysql string literals treat backslash as an escape
+	// character (so the SQL needs '\\'), sqlite ones do not (so it needs
+	// '\', and has no default escape character at all).
+	likeContains(col, substr string) (predicate string, arg string)
+}
+
+// escapeLike backslash-escapes LIKE pattern metacharacters in s.
+func escapeLike(s string) string {
+	r := strings.NewReplacer(`\`, `\\`, `%`, `\%`, `_`, `\_`)
+	return r.Replace(s)
+}
+
+type sqliteDialect struct{}
+
+func (sqliteDialect) name() string         { return "sqlite" }
+func (sqliteDialect) datetimeType() string { return "TEXT" }
+
+func (sqliteDialect) upsert(insert string, cols []string) string {
+	sets := make([]string, len(cols))
+	for i, c := range cols {
+		sets[i] = c + "=excluded." + c
+	}
+	return insert + " ON CONFLICT(id) DO UPDATE SET " + strings.Join(sets, ", ")
+}
+
+func (sqliteDialect) createIndex(ctx context.Context, db *sql.DB, index, table, column string) error {
+	_, err := db.ExecContext(ctx,
+		"CREATE INDEX IF NOT EXISTS "+index+" ON "+table+" ("+column+")")
+	return err
+}
+
+func (sqliteDialect) likeContains(col, substr string) (string, string) {
+	return col + ` LIKE ? ESCAPE '\'`, "%" + escapeLike(substr) + "%"
+}
+
+type mysqlDialect struct{}
+
+func (mysqlDialect) name() string         { return "mysql" }
+func (mysqlDialect) datetimeType() string { return "DATETIME(6)" }
+
+func (mysqlDialect) upsert(insert string, cols []string) string {
+	sets := make([]string, len(cols))
+	for i, c := range cols {
+		sets[i] = c + "=VALUES(" + c + ")"
+	}
+	return insert + " ON DUPLICATE KEY UPDATE " + strings.Join(sets, ", ")
+}
+
+func (mysqlDialect) createIndex(ctx context.Context, db *sql.DB, index, table, column string) error {
+	var n int
+	err := db.QueryRowContext(ctx,
+		`SELECT COUNT(*) FROM information_schema.statistics
+		 WHERE table_schema = DATABASE() AND table_name = ? AND index_name = ?`,
+		table, index).Scan(&n)
+	if err != nil {
+		return err
+	}
+	if n > 0 {
+		return nil
+	}
+	_, err = db.ExecContext(ctx, "CREATE INDEX "+index+" ON "+table+" ("+column+")")
+	return err
+}
+
+func (mysqlDialect) likeContains(col, substr string) (string, string) {
+	return col + ` LIKE ? ESCAPE '\\'`, "%" + escapeLike(substr) + "%"
+}